Immunology and Human Body (With Diagram)

Let us make an in-depth study of the immunology of the human body. The below given article will help you to learn about the following things: 1. Immunology of the Human Body 2. Antigen 3. Antibodies 4. Antigen-Antibody Reaction 5. Immunity 6. Acquired Specific Immunity (Specific Resistance) 7. The Immune Response 8. Allergy (Hypersensitivity) and 9. Autoimmunity.

Immunology of the Human Body:

The study of immune responses of the human body is known as Immunology.

History of Immunology:

The inhabitants of East Africa have succeeded in vaccination against the bite of poisonous snakes from time immemorial, they could immunise artifi­cially against tick borne relapsing fever and could protect their cattle from contagious pleuropneu­monia by their old method of vaccination.

In 2,000 – 3,000 B.C., children were vaccinated against small pox in South East Asia by the folk methods of direct contact of hands of the children. Only after 100 years after Jenner’s work, Immunology was enriched with his own methods built on the scientific works of Louis Pasteur.

Antigen:

Such substances (egg white, serum, milk, snake venom, dead and living bacteria, bacterial exotoxins and endotoxins, plant or animal tissue) containing proteins may act as antigen when introduced into the deeper tissues of the body. Protein taken in the gastrointestinal tract as food may not act as antigen under normal conditions because they are rapidly destroyed by the gastric juice.

To act as antigen, a protein must gain entrance to the blood and other tissues in a chemically unaltered state. Entrance is commonly by injection or infection (i.e., by routes other than the gastrointestinal tract) or by parenterally.

An antigen (Gr. anti. against genosgenus) is a foreign protein (organic substances of colloidal struc­ture) which when introduced parenterally (Gr. para, beyond or outside of; enteron, intestine), by route other than the gastrointestinal tract into the human body, it will provoke the lymphoid tissue of the hu­man to produce antibodies which will react specifi­cally with the same antigen and not with other sub­stances.

Antigens have the following main properties:

(a) Antigenicity (the ability to cause the pro­duction of antibodies);

(b) Antigenic specificity (the ability to enter into an interaction with the correspond­ing antibodies).

The small portion of a molecule may be called as unit of antigen. This unit is known as antigenic determinant. This determinant is a contour which locks one molecule of antibody. As antigenic determinants have a three-dimensional structures, the antibody molecule can approach or can get attached to any of the wide range of possible configurations in a single determinant (Fig. 21.1).  The specificity of the antigen is dependent upon antigenic determinant (epitope), i.e., chemical group­ings of the antigen molecules. When the antigen comes in contact, the animal reacts. This is known as “acquired immune response”.

The acquired immune response may be of two forms:

(a)The humoral or circulating antibody response;

(b) Cell mediated re­sponse.

An antibody directed against an antigenic determinant of a particular antigen molecule will re­ad only with this determinant or another very similar structure. Even minor chemical changes in the deter­minant will reduce the ability of the original antibody to react with the altered determinant. Antigens which are capable of stimulating an immune response is known as Immunogens.

Antigens may be complete or partial:

Complete antigens are substances which cause the production of antibodies and react with them in vivo and in vitro (e.g., foreign proteins, bacteria, tox­ins, rickettsiae, viruses). Partial antigens are known as haptens. They do not produce antibodies but react with the antibodies produced by another substance. They are lipids, com­plex carbohydrates and other substances of low mo­lecular weight. The addition of protein to haptens in small quantity will convert the hapten into complete antigen. In this case, the protein carries out the func­tion of a conductor.

The antigenic function of bacteria, rickettsiae and viruses is characterised by species and type specificity. Salmonella typhiantigen induced immu­nity against Typhoid and it will not produce immunity against Paratyphoid fever. So this antigen is species and type specific. The type specificity is associated with the presence of special polysaccharide com­plexes in the bacterial cell.

In the laboratory, there is cross-reaction between antisera to certain bacterial antigens and antigens present in cells such as erythrocytes. These antigens are known as heterophil antigens. Antisera to such antigens will cross-react with cells of different spe­cies of animals and various microorganisms.

Chemi­cal groupings of heterophile antigens are not yet known. The best known of the heterophile antigens is the Forssmann antigen which is present in the red cells of many species as well as in the bacteria such as pneumococci and salmonella. Another heterophile antigen is found in Escherichia coli and human red cells of Group B.

Semihaptens”(iodine, bromine, quinine, antipyrin, colloidal iron, azoprotein) are not antigenic them­selves, but in combination with the protein of the body, they acquire the properties of complete anti­gen. All natural proteins are inherited with the prop­erties of chemical, structural and functional specificity. Proteins of different species of animals, plants, bacte­ria, rickettsiae and viruses can be differentiated by immunological reactions.

Chemical Nature of Antigens:

Antigens may be proteins, polysaccharides or lipids. Antigens of cell wall of bacteria (streptococci, staphylococci, corynebacteria) are rather less well de­fined and appear to be complex mucopeptides, some­times associated with lipids. Capsulated microorgan­isms (pneumococci) have complex high molecular weight polysaccharides in their capsules.

General Properties of Antigens:

1. Antigen should be foreign, egg albumin is an excellent antigen in the rabbit, but fails to produce an antibody response in fowl;

2. More foreign, it is more powerful antigen;

3. It should have a molecular weight of 5,000. Sometimes, some substances of low molecular weight (aspirin, penicillin, sulphonamides) can act as’ antigen when applied to the skin.

4. The ability of antibody to form a strong bond with an antigen depends on intermolecular forces which act strongly only when two molecules come in contact in a very precise manner. The better they fit the stronger the bond.

Isoantigens:

Isoantigens are those substances which have antigenic properties and are contained in some individuals of a given species. They are found in erythrocytes of animals and men. At first it was established that there are two antigens (A and B) in the human erythrocytes and in the sera, there are two antibodies (anti-A and anti-B). Only heterogenic antigens and antibodies (agglutinins) are found in hu­man blood.

These combinations may be represented as below:

On the basis of antigenic structure, the erythro­cytes of all people can be divided mainly into four groups. Consequently, variants of antigens of eryth­rocytes in the second (A) and fourth (AB) groups were isolated. The Group A consists of two sub-groups A₁ and A₂.

The AB groups containing A₁ B and A₂ B and the antigens M and N and M₂ and N₂ etc. have been re­vealed. At present 30 antigens are known. Besides, there is rhesus factor in the erythrocytes. All these groups are taken into account during blood transfu­sion.

Auto antigens are those substances (eye lens, spermatozoa, skin, kidney, lung) which do not, under normal condition, come in contact with the immune system of the body, then the antibodies are not pro­duced against such cells and tissues.

However, if these tissues are damaged, then the auto antigens may be released and may cause the production of antibodies which react with the corresponding auto antigens of the cells resulting in the development of glomeru­lonephritis, interstitial thyroiditis, orchitis, corneal opac­ity).

The cooling, radiation, drugs (amidopyrine, sul­phonamides), virus infection (virus pneumonia, infec­tious mononucleosis, bacterial proteins, toxins of strep­tococci, staphylococci, tubercle bacilli) and other fac­tors may release auto antigens from the tissues.

Antibodies (Immunoglobulin’s):

History of Antibodies:

Von Behring and Kitasato in Berlin in 19th cen­tury recognised antibodies in the blood stream of an immunized animal against tetanus and diphtheria they observed clumping or agglutination of microor­ganisms and precipitation of soluble antigens by the serum of the immunized animal. They practiced also the serotherapy method.

Serotherapy is the injection of antiserum, for the therapeutic use, from an immunized animal to non-immune patient. They also carried out the frag­mentation of immunoglobulin’s by pepsin digestion. Antibodies are synthesised under the influence of antigens which penetrate into the body and dis­turb the normal blood composition. Antibodies are specific substances in the bodies of the vertebrates secreted in the tissue fluids from the lymphoid cells that have been stimulated by foreign substances (an­tigens) with which they react specifically.

Antibodies appear in the blood due to the in­fection or immunization by live (attenuated) or dead bacteria, rickettsiae, viruses, toxins or toxoids etc. An­tibodies which occur under the influence of active immunization are named immune antibodiesin con­trast to normal antibodies which are found in the sera of men and animals which had not been infected or exposed to immunization. Normal and immune anti­bodies can render harmless the causal agents of in­fectious diseases.

Nature of Antibodies:

1. Antibodies are globulins which have been altered under the influence of antigens;

2. Molecules of antibodies, like normal serum globulins are probably asymmetrical;

3. Antibodies are found in globulin fractions;

4. The transformation of normal globulin into immune globulin is due to alteration in the spatial configuration of active atomic groups of the protein molecules. A change in protein metabolism is respon­sible for this process. The mechanism of antibodies production is not yet clear, though many theories are put forth;

5. Antibodies do not differ greatly from normal globulins;

6. They have almost similar isoelectric points, viscosity molecular weight (from 100,000 to 1,000,000) and are sensitive to the temperature and to other denaturating agents.

7. Antibodies are thermolabile and denatured on heating at 70°C for one hour;

8. The pH of the medium and other factors af­fecting proteins can also affect the activity of anti­bodies;

9. Antibodies are not denatured by precipita­tion with ethyl alcohol at low temperature (from 0 to +4°C) but are denatured by alcohol at high tempera­ture. Natural salts (magnesium sulphate, ammonium sulphate and sodium sulphate) can cause the pre­cipitation of proteins, but do not denature antibodies. Ethyl alcohol at low temperature and natural salts can be used for fractionation of immune sera to obtain them in a pure state.

There are five types of human immuno glo­bulins: IgG; IgM; IgA; IgD and IgE, on the basis of polypeptide chain structure consisting of heavy chains and light chains joined by disulphide bond. Immunoglobulin G or IgG.

Seventy five per cent of total serum constitutes IgG. It is the major immu­noglobulin component of serum and has a molecular weight of 150,000 in man. The molecule has two antigen combining sites which are called as Fab (an­tibody binding) portions of the molecule.

Fab frag­ment has both light and heavy chains. Light chains are of two types. They are known as K or L (K, Kappa or L, Lambda) chains. The heavy chain or V chain exists in four forms: lgG₁, lgG₂; lgG₃ and lgG₄.The molecule has also Fc fragment for complement fixation.

IgM, IgA, IgD and IgE:

Like IgG globulin, each of these classes of immunoglobulin’s in man contain K and L light chains. The heavy chains are unique for each of the types of immune globulins. IgM contains (i chain, IgA alpha chain, IgD 8 chain and IgE e chain.Structure and Function of Immunoglobulin’s:

In the Fab portion, the heavy chain component (Fd portion) contains as much as 85 per cent of the antigen-binding ability. The light chain seems to act together with the heavy chain to form a stable anti­body-combining site. The Fc portion of the IgG mol­ecule is responsible for activation of complement sys­tem.

On combination with antigen, the IgG molecule springs open at the hinge region exposing the Fc portion which can activate the complement. If IgG molecule has not combined with antigen, it will be unfolded. A single IgM molecule can get attached to a red cell by multiple combining sites (Fig. 21.2) and can bring about lysis; but for the same effect, 1,000 IgG molecules are required.

Agglutination is due to linkage of a particulate antigen (red blood cell or bacteria) by two Fab frag­ments of IgM. The IgM is of large size, so it is confined in the blood stream and plays an important role in the protection against blood invasion by microorganisms. IgM deficiency may result into septicemia.

Diphtheria toxin, lysozyme or virus (poliovirus) can be neutralized by IgG antibodies and IgG antibodies can neutralize more effectively than IgM antibodies. Fc frag­ment of IgG molecule may assist the Fab fragments in neutralisation of virus infectivity and also helps in the selective transport of IgG.

The characteristic fea­ture of IgE class immunoglobulin is responsible for the hypersensitivity reaction in man which is due to the activity of Fc portion of IgE molecule. IgM anti­body specific for particulate antigen (bacteria) makes the antigen more susceptible to the phagocytosis by coating the surface of particulate antigen, so it is called as opsonizing antibody (Opsonin).

Selective Transport of IgG:

In pregnant women, IgG globulins can pass through the placenta and reach the foetal circulation; this process is not due to the mere filtration but due to the selective transfer of IgG molecules which is brought about by a part of Fc fragment of IgG heavy chains.

This property is a characteristic feature only of the y chain of IgG and is not found in the n chain of IgM and a chain of IgA. This mechanism of selective transport is mainly observed in primates. Similarly, immunoglobulin’s are absorbed from the colostrum through the intestinal epithelium of the ruminants. IgA globulins are selectively secreted into sa­liva, respiratory, intestinal mucous secretions and into the colostrum. This is another mechanism of selec­tive transport.

There are two types of IgA:

Serum IgA and dimer lgA. Dimer form of IgA is manufactured locally and has an attached secretory transport piece which is not found in serum IgA. This transport piece is added to the IgA molecule during its passage into the mu­cous secretions from the lamina propria of the intes­tinal and respiratory tract. This secretory IgA would protect from the infection. The dimer form of IgA acquires the ability to fix the complement after at­taching to Gram negative organisms.

The monomeric form of IgA can also pass from the lamina propria to the blood stream via lymphatic’s so that the infection of the intestine may lead to in­creased serum IgA levels. Local application of vac­cine may stimulate dimer IgA.

Types of Antibodies:

Opsonins:

These antibodies render the micro­organisms more susceptible to phagocytosis by al­tering the surfaces of the antigens.

Cytolysins:

They help to dissolve the cells that stimulated their production. An toxins are formed due to the stimulation of exotoxins which are protein in nature and soluble in water. These antibodies are inactivated or neutralized by the exotoxins.

Hemolysins:

If a rabbit is injected several times with minute quantity of washed erythrocytes of an­other species of animal, e.g., sheep, the serum of the recipient rabbit acquires the property of destroying the erythrocytes of the sheep.

This is due to the for­mation of cytolysins that sensitize the sheep red blood cells to complement and cause the lysis of the sheep erythrocytes. The hemoglobin escapes into the sur­rounding fluid. This process is called hemolysis. The haemolytic cytolysins are called haemolysins.

Precipitins:

These antibodies combine with the molecules of soluble proteins and bring about a cloudiness in the fluid or visible flakes. This flocculent turbidity is known as precipitate as seen in Venereal Disease Research Laboratory (VDRL) test; they may form line of precipitation when they react with solu­ble antigens.

Agglutinins:

They bring about the clumping of particulate antigens (bacteria, red blood cells) — hence agglutination. Many diagnostic tests are based on bacterial agglutinins. For example, Widal test is used to diagnose Typhoid fever. Hemagglutinins are responsible for the aggluti­nation of red blood cells or hemagglutination; cold hemagglutinins; viral hemagglutinins; and isohemagglutinins.

(a) Cold hemagglutinins are substances that appear in the blood of person with cer­tain respiratory diseases, e.g., atypical pneumonia of unknown or viral origin and trypanosomiasis. The sera of the patient in dilution of 1:10,000 agglutinate their own erythrocytes (auto-agglutination) when cooled to 2°C; but not at 37°C.This agglu­tination at low temperature is known as cold hemagglutination.

(b) Viral Hemagglutinins:

Agglutination of red cells is brought about by several respira­tory virus (Influenza virus) not by antibody. The virus itself attaches to the red cells of the chick at 5°C and agglutinate them. It can be separated (eluted) from them by mixing with saline solution followed by an incubation at 37°C.

The viral hemagglutination tests are useful in the study of viruses and the diagnosis of viral diseases. In some infections, antibodies may prevent the viral agglutination of erythrocytes. They are called as hemagglutination-inhibition (HI) antibod­ies.

(c) Isohemagglutinins:

These antibodies are formed in response to one or more anti­gens (A, B, Rh, M) found in human erythrocytes. They do not agglutinate one’s own red blood cells, but cause agglutination, at body temperature (37°C), of red blood cells of another person. They play an im­portant role in blood transfusion.

If the serum of the recipient agglutinates the red blood cells of a donor or vice versa, then there is possibility of reaction or death of the recipient. It is not necessarily that the sera of all donors should agglutinate the erythrocytes of all the recipients and vice versa, because there are numerous classes or groups of blood with respect to isohemagglutinins, called blood groups.

Blood Groups:

One major system of blood group­ings consist of A, B, AB and O groups. It detects the presence or absence of the agglutinogens (antigens) in the erythrocytes of the persons to be identified and grouped. For example, anyone person may have antigen A (Group A) or B (Group B) or both (Group AB) or neither (Group O). A person’s own serum never contains agglutinins against his own Erythrocytes, but only against the absent agglutinogens.

A person’s blood group is a physiological con­stant determined by the basic Mendelian laws of in­heritance. It is possible to avoid the hemagglutination, by properly selecting donors and recipients of com­patible group. Blood banks collect citrated blood do­nations already grouped and store in the refrigerator until needed.

Blood Grouping:

A nurse should have the knowl­edge of grouping the blood. A drop of the patient’s (recipient) blood should be added to a small amount of physiological saline (0.9 per cent sodium chlo­ride) on a microscopic slide and then mixed with rab­bit sera containing pure anti-A and anti-B antibodies. A readily visible agglutination will appear of the pa­tient (recipient). If the cells of the patient (recipient) are aggluti­nated by both anti A and anti B. rabbit sera, then the cells of the patients are of group AB.

If the cells are agglutinated by neither sera, they are of Group O. If anti A serum agglutinates them and anti B serum does not, then the blood cells are of Group A; if anti B serum agglutinates them and anti-A serum does not, the cells are of Group B. Certain irregularities in reaction can be elimi­nated and the blood groupings can be ascertained by testing the serum of the recipient against the red cells of the donor.

Hemagglutination in Blood Grouping:

Anti B serum and anti A serum have been mixed on a glass slide with erythrocytes of Groups 0, A, B and AB. Anti B serum agglutinates erythrocytes of persons of Group B and AB; anti A serum agglutinates erythrocytes of persons of Groups A and AB.

Universal Donor:

A person of Group O is some­times called as Universal donor because his red blood cells are not usually agglutinated by serum of recipi­ents of any other groups; but he must receive only Group 0 blood because his serum agglutinates cells of all other groups in the A-B-0 system.

Direct cross-matching between recipient and donor is necessary to have a “safe universal donor” There are several other antigen antibody systems in human bloods, e.g., Rh factor, M-N-S, P types which may cause serious transfusion reactions irrespective of A, B, O or AB groups.

Rh Factor:

When erythrocytes of Rhesus (Rh) monkeys are injected into rabbits, the serum of the rabbits contained anti-Rh agglutinins which aggluti­nate erythrocytes of 85 per cent or more human be­ings. The antigen in the human red cells which re­acted with anti-Rh rabbit serum was designated as the Rh agglutinogen.

The person having the Rh ag­glutinogen is called as Rh positive (+). If the Rh agglu­tination is absent in red blood cells of the remaining 15 per cent whose red blood cells do not react with the anti-Rh rabbit serum, they are called as Rh-negative. The Rh agglutinogen is complex and corresponds to several antibodies or blood factors – hr, br’, rb’, br’, rh’. The most important is Rh factor.

Rh Factor and Transmission:

If an Rh negative person receives a blood transfusion from a person who is Rh positive, the recipient develops in about two weeks antibodies that agglutinate the red cells of all Rh positive persons. Now, if a subsequent transfu­sion is given from an Rh positive donor and the Rh antibody titre (concentration) will rise to maximum, thereby all the red cells of the donor are agglutinated and ultimately haemolysis leading to severe transfu­sion reaction and death. Therefore, it is essential — before each blood transfusion — to evaluate the Rh factor and blood group.

Erythroblastosis foetalis (Hemolytic disease of the new born). If Rh negative woman becomes preg­nant by an Rh positive man, the child will most prob­ably inherit Rh antigen in its blood cells. During preg­nancy, if the child is Rh-positive (i.e., Rh factor), fetal red blood cells with Rh antigen often cross the foetal barrier, pass into the mother’s blood circulation and the foetal Rh factor stimulates the maternal tissue to produce Rh antibodies.

These Rh antibodies diffuse back into the foetal blood circulation. In any initial pregnancy, since the mother’s Rh antibody titre is too low, there will be no detrimental effects in the foe­tus; when Rh antibody concentration increases with subsequent pregnancies, the red cells of the foetus are destroyed by Rh antibodies.

Hence, the child with Rh factor (i.e., Rh positive) and Rh antibodies may be born dead (uncommon) or with hemolytic disease of the new born (erythroblastosis foetalis) which is common. This type of condition may prevail if Rh negative mother has received a blood transfusion from Rh positive donor. The periodic check-up of the maternal blood for rising titre of Rh agglutinins is of great importance as a precautionary measure.

Antigen-Antibody Reaction:

The interaction of the antigen and antibody is a chemical reaction and is specific. The molecules of the antigen and antibody bind with their terminal groups (Fig. 21.1) and form firm complexes. When molecules of antibody and antigen are brought to­gether in solution, they interact with each other by the formation of a link between an antigen binding (ab) site on the immunoglobulin molecule (part of the Fab fragment) and the particular chemical group­ings which make up the antigenic determinant of the antigen molecule.

The molecules are held to­gether by intermolecular forces which are effective only when the antibody combining site and the antigenic determinant group are able to make close contact. The study of the reaction between the antigen and antibody is known as “Serology”

Serology is classified into:

1. Precipitation;

2. Agglutination;

3. Complement fixation;

4. Neutralisation;

5. Immobilisation;

6. Intradermal reaction.

1. Precipitation, in which the reaction takes place between antibody and antigen molecules which are soluble in nature; it is specific, sensitive and can detect even diluted antigen (1: 1,000,000 – 110,000,000). VDRL, Kahn test for syphilis; gel diffu­sion test; counter immuno-electrophoresis.

2. Agglutination, in which there is reaction be­tween antibody and particulate antigen (erythrocyte, bacteria) in the presence of electrolyte (0.85 per cent sodium chloride solution) in definite proportions. (Coombs ‘test, blood groupings; Widal test; Weil-Felix test; tube agglutination test for Brucellosis);

Coombs’ test:

The Coombs ‘test (anti-globulin test) is used, in particular, to detect incomplete agglutinins in rhesus negative mothers. To determine the fixation of agglutinins by the patient’s erythrocytes, anti-globulin serum is added which, in saline solution, is capable of causing marked agglutination of erythrocytes sensitized by incomplete agglutinins. A molecule of anti-globulin binds two molecules of incomplete agglutinins fixed to two different erythrocytes, due to which the agglutination reaction takes place.

3. Complement fixation reaction, in which the antibody molecules, after reaction with antigen acti­vate the serum complement which gets fixed with the antibody antigen complex — Wassermann test for syphilis.

4. Neutralisation testis used in virus identifica­tion; Antitoxin-toxin neutralisation test (in tetanus).

5. Immobilisation test for motile bacteria;

6. Intradermal test for the reaginic antibody characteristic of skin hypersensitivity.

Immunity:

The term immunity (Lat.immunis—freed from homage, save from something) means resistance of the body to the pathogenic microbes, their toxins or to other kinds of foreign substances. In other words, it can be defined as the body resistance power to in­fections immunity.

1. Natural (innate),

2. Acquired.

1. Natural immunity (non-specific resistance) is present from birth. The innate resistance mechanisms are non-specific in the sense that they are effective against a wide range of infective agents. The innate immunity is genetically controlled.

Determinants of Natural Immunity:

(a) Species and Strains:

Different species of animals are susceptible to different dis­eases, e.g., the guinea pig and man are not susceptible to diphtheria, whereas the rat is susceptible to diphtheria. Only man is susceptible to syphilis, gonorrhea, ty­phoid. Rabies and Brucellosis affect both men and animals. American Indian and the Negro are genetically more suscepti­ble to tuberculosis than Caucasians.

(b) Individual Difference and Influence of Age:

Sickle cell anaemia (sickling) disease is genetically controlled. Sickle cell anae­mia patients are resistant to malaria, as Plasmodium falciparum cannot parasitise red blood cells of sickle cell anaemia. Because of the immaturity of the immunologi­cal mechanisms affecting the lymphoid system, chil­dren and young animals are highly susceptible to in­fections, older persons have diminished resistance as a result of the probable waning of the activity of im­mune response.

(c) Hormonal Influences, Sex. Patients of dia­betes mellitus, hypothyroidism, adrenal dysfunction have decreased resistance to infection. The reasons are not yet well pro­pounded. Sexes will not show the marked differences.

(d) Nutritional Factors:

Inadequate diet may be associated with the increased susceptibility to various bacterial diseases. It has been experimentally proved that when a large number of animal species were undernourished, they become less susceptible than normal animals to a vari­ety of viruses including vaccinia virus and certain neurotropic viruses, e.g., poliovirus. The same condition may be observed in case of malaria. When the nutrition level is lowered, there may be deficiency of paraaminobenzoic acid essential for the multiplication of malarial parasites.

Mechanism of Natural Immunity:

(a) Mechanical barriers and surface secretion,

(b) Bactericidal substances of the tissues and body fluids:

(i) Lysozyme,

(ii) Basic polypeptides.

(c) Phagocytosis and Inflammation:

(i) Phagocytin,

(d) Temperature,

(e) Complement system.

(a) Mechanical Barriers and Surface Secretions:

1. The intact skin and mucous membrane of the body give protection against non-pathogenic or­ganisms and a high degree of protection against pathogens. Because of the horny layer of the skin consisting mainly of keratin, which is not digestible by most organisms and protects the living cells of the epidermis from the organisms and toxins, the skin is a most resistant barrier.

2. Sebaceous secretions and Sweat of skin con­tain bactericidal and fungicidal fatty acids and have a protective mechanism. The microorganisms are trapped by the sticky, mucous secretion covering the mucous membrane of the respiratory tract. They are swept down by the cilia or hair-like processes of the mucosa along with the mucous secretion towards the oropharynx and are swallowed. In stomach, most of the microorganisms are destroyed by the acid se­cretion.

Nasal secretion and saliva containing mucopolypeptide inactivate some viruses. The tear and mucous secretions of respiratory, alimentary and genitourinary tract contain lysozyme which is active against some Gram-positive bacteria.

(b) Bactericidal Substances and Body Fluids:

(i) Lysozyme is a basic protein present in leucocytes and body fluids except cer­ebrospinal fluid, sweat and urine. It splits the sugars of the cell wall of Gram-posi­tive bacteria and causes lysis of bacteria. Egg white and human tear are rich source of lysozyme.

(ii) Basic Polypeptide is basic protein derived from the damaged tissue and blood cells. It has a bactericidal activity.

(c) Phagocytosis and Inflammation:

When the microorganisms invade the blood stream or tissue fluids, they are engulfed rapidly by various circulating and tissue fixed phagocytic cells.

These cells are of two types:

(a) Polymorph nuclear leucocytes or microphages of the blood;

(b) Mononuclear leucocytes or macrophages, distributed throughout the body. The cells fixed in the tissues are the cells of the reticuloendothelial layer lining liver si­nuses (Kupffer cells), red bone marrow, spleen and lymph node.

Macrophages in the blood are known as monocytes, those in the connective tissue are known as histiocytes, those in spleen, lymph and thymus are known as the sinus lining macrophages (called as littoral cells). Inflammation is a defensive response of living body tissues to any irritating or injurious agent.

The inflammatory reaction is a complex process involv­ing several physiological process which:

(1) Removes the cause of the irritation and

(2) Repairs the damage, i.e., to heal the wound or lesion.

The inflammation may be due to:

(a) Mechanical factors (trauma, cut),

(b) Chemical agents (acids, excessive appli­cation of mustard, gases, bee stings),

(c) Physical agents (excessive heat, cold, ul­tra violet rays),

(d) Living agents (microorganisms, worms).

Four cardinal sings of inflammation are:

1. Rubor (redness, hyperemia);

2. Tumor (swelling);

3. Calor (eat);

4. dolor (pain) can be observed in a boil or in an inflammed area. The increased amount of blood in the dilated local blood vessels may cause the redness; the compression and injury of the sensory nerve branches in the tissue may result into pain.

The dila­tation and increased permeability of the blood vessels resulting in the collection of fluid under slight pressure in the spaces between the tissue cells is responsible for swelling (often called edema— inflam­matory exudation of fluid).

Hyperemia is due to increased amount of blood in an inflammed area. Thrombi or clots are formed in the tiny vessels all around the point of injury due to the slowing down of blood flow in that particular area and prevent the entry of microorganisms into the general circulation.

Pus Formation:

When the blood flow slows down or stops, the leucocytes begin to migrate in between the cells of the vessel walls by amoeboid motion called diapedesis. The leucocytes become pus cells. In intense inflammation as in gonorrhea, much pus is formed and discharged, but a mild inflammation may heal without pus formation.

Temperature Changes:

The inflamed area is usu­ally hot compared to the temperature of the body surface which may be due to the warmth of the deep body blood brought rapidly to the body surface or warmth due to the chemical changes in the infected area.

Defensive of Fibrin:

The inflammatory area is filled and surrounded by a continuous network of fibrin fibres. At the periphery of the infected area, connective tissue cells (fibroblasts) begin to grow among these fibres. The acute inflammatory area is therefore soon surrounded by a wall of fibrin that tends to confine the inflammatory process and pre­vent the spread.

A closed sac formed by pyogenic membrane, is filled with pus, microorganisms and walled off form­ing abscess, pimple,. boil, carbuncle. Several inter communicable abscesses are called carbuncle. Enzymes produced by various microorganisms digest the wall of the abscess nearest to the exterior finally making an opening through which the abscess drains. By this process, the boil heals.

Types of Inflammation:

(a) Acute inflammation is of short duration (for few days).

(b) Chronic inflammation is of long duration (weeks, months, years).

(c) Catarrhal inflammation is of mild type seen in the throat, nose in cold or influ­enza.

(d) In serous inflammation, the serum like fluid is more and the fibrin is less. If pus formed is more, the process is described as purulent or suppurative. A fluid may be sanguinopurulent or fibrinopurulent.

Healing and Scar Formation:

As soon as the in­fection or injury is brought under control by fibrin formation and other body defensive mechanism, tis­sue cells (fibroblasts) and blood vessels start to grow into the area of healing from periphery and form new healthy tissue. This early healing process is known as Organisation. The tissue thus formed is solid, tough connective tissue. This tissue constitutes scar tissue and this process is known as cicatrisation. When there is inflammation, there is a response, there is initial polymorph infiltration; bacteria are di­gested and disintegrated within one hour or two, prob­ably due to acid and digestive enzyme of phagocytes.

Phagocytin:

Polymorphs contain acid soluble protein (phagocytin) which is bactericidal to Gram- negative bacteria.

Temperature:

Mycobacterium marinum, which infect cold blooded animals, cannot infect man. Anthrax bacilli cannot infect fowls because of fowl’s high tempera­ture, 107°C. If the temperature of the fowl is lowered, it can infect fowls. High temperature (above 40°C) can kill Gonococci and Treponema.

Complement System:

Complement is a heat labile serum component which has the ability to lyse red blood cells and de­stroy Gram-negative bacteria. It is an extremely com­plex group of serum proteins present in low concen­tration in normal serum. Components of complement are C₁, C₂, C₃, C₄, C₅, C₆, C₇, C₈, C₉.The complement is responsible for the biological activity of complement fixing antibodies. Recently, a new pathway for complement acti­vation has been described. C₁, C₄ and C₂ components are not involved.

The activation starts at C₃ and pro­ceeds to the later components:

Function:

1. Complement action is of considerable im­portance because of its ability to get fixed with vari­ous types of cells such as Gram-negative bacteria or human red blood cells after they have been inter­acted with specific antibody; and

2. Renders bacteria susceptible to lysozyme by digesting holes in the polysaccharide which protects inner lysozyme sensitive layer of mucopeptide in the cell wall;

3. complement also plays an important role in the attraction of polymorphonuclear phagocytes to the sites of antigen – antibody interaction.

This chemo- taxis appears to depend on the components of C₅, ₆, C₆ C₇ interacting with C₁, C₄, D₂, C₃ complex.

Acquired Specific Immunity (Specific Resistance):

Microorganisms that overcome the innate non­specific resistance are faced, after an interval, by the host’s second line of defense, i.e., specific immune response. Acquired immunity is specific in the sense that it protects against a particular pathogen or its toxic product.

The specific immune response takes two forms:

(a) Humoral immunity depends on the ap­pearance of globulins or antibodies in the blood. These antibodies combine specifi­cally with antigens that stimulated their production. Examples, antigen molecule or particles of bacteria may be clumped, their toxins may be neutralized and sub­sequently they are ingested and digested by phagocytes. Cells are lysed by the acti­vation of complement.

(b) Cell mediated immunity depends upon the development of lymphoid cells which are specifically sensitized to the inducing agents and which react directly with the antigen to bring about cytotoxic effects. The function of lymphoid tissue is not yet well understood.

Actively acquired immunity is due to:

1. Infectious agents which may be of two types:

(a) Some infections — diphtheria, whooping cough, small pox and mumps induce a long lasting immunity,

(b) Some infections like common cold and influenza confer immunity lasting for a short time.

2. Vaccination is an artificial process which stimulates the lymphoid tissue and is adapted to meet practical requirement. It is under human control. Sev­eral such artificial processes are used for artificial im­munity. Vaccines are cheapest and most effective means of controlling deadly infectious diseases.

(A) Vaccine Prepared From Living Attenuated Organisms:

Active specific immunity may be acquired arti­ficially similar to subclinical infection, by means of injection of living attenuated (weakened) organisms in the human body which cannot cause a severe in­fection, e.g., Bacille Calmette Guerin (BCG) vaccine; small pox vaccine; Anti Rabies vaccine — vacca, (Latin for cow).

1. BCG Vaccination:

Calmette and Guerin, French scientists, developed a BCG vaccine consist­ing of live tubercle bacilli of low virulence for human being as an immunizing agent against tuberculosis.

Cholera vaccination. A live oral cholera vaccine ‘s recently used.

2. Small pox vaccination:

Small pox vaccination is carried out by in­oculation of living cowpox virus into the human body which could give protection against small pox virus because of their antigenic relationship. The vaccine was available in freeze dried form, when small pox was in existence. Now small pox is completely eradi­cated.

3. Anti-Rabies Vaccine:

Sample vaccine is pre­pared from the brain of sheep infected with the fixed rabies virus. Infected sheep brain suspension inacti­vated by 0.5 per cent phenol vaccine may cause pa­ralysis in small proportion (1: 4,000 to 1:10,000) of human beings, because of the presence of nerve cells in the vaccine (allergic type of encephalitis).

4. Avinised Rabies Vaccine:

To avoid the nerv­ous symptoms, the vaccine is prepared from virus cultivated repeatedly in successive live embryonated avian eggs, inactivated by formalin, ultraviolet irradia­tion or phenol and finally the vaccine is dehydrated. This vaccine is reconstituted, used safely and effective­ly than sheep brain tissue vaccine. Human Diploid Cell Strain (HDCS) vaccine is highly antigenic free from side effects and is costly. It is a recent vaccine against Rabies.

(B) Vaccine Prepared From Dead Organisms:

The active artificial immunity can be produced by another method, i.e., by injecting the actual or­ganisms that cause the disease into the body. The body reacts and produces the antibodies. In this method, the organisms are dead or killed by heat or chemical disinfectant.

Many types of bacteria vaccines are used. Ty­phoid vaccine and Whooping cough (Pertussis) vac­cine are very common. These vaccines are prepared by submitting the bacterial suspensions to a heat of 60°C for 1 hour to kill the living bacteria. It is cus­tomary to give three successive injections of the vaccines at intervals of one month and an annual booster dose.

1. Mixed Vaccines:

Vaccines containing more than one kind of organisms is known as mixed vaccines, e.g., Triple vaccine (Tetanus, Pertussis, Diph­theria, TPD) vaccine is not only effective, but it saves many extra pricks.

2. Sensitized Vaccines:

Organisms meant for the preparation of vaccine are sometimes treated before use with the serum of a person or animal im­mune to the bacteria. These bacteria combine with the antibody in the serum, so that they are acted upon quickly by the blood, tissues or phagocytes of the person into whom they are infected and ultimately immunity is produced rapidly and effectively. Such vaccines are called as Sensitized vaccines.

3. Viral Vaccines:

Killed viral vaccines are the vaccines against Influenza and Salk vaccine against Poliomyelitis. Live viral vaccine is Sabin Oral Poliomy­elitis vaccine.

(C) Injection of Modified Bacterial Exotoxin:

1. Toxoid:

Certain bacteria damage the body tissue by elaborating the toxins. When these toxins are detoxified by formaldehyde, they can be used as immunizing agents against the diseases, e.g., Tetanus toxoid (TT) against tetanus; diphtheria toxoid against diphtheria.

Passively Acquired Immunity:

Administration of immune serum from another species, e.g., horse – diphtheria antiserum, tetanus antiserum (ATS) and gas gangrene antiserum were used therapeutically until the advent of antibiotics. Still now, it is used. Passive immunity due to maternal antibody may be trans­ferred to foetus by the passage of maternal antibody across the placenta in some species — man and rab­bit – where a particular part of immunoglobulin polypeptide chain has been found necessary for ef­fecting the transfer.

In other species, such as dogs and rats, antibodies are transmitted through the co­lostrum via the intestine and via placenta; the other animals, lamb and calf receive this form of immunity only by means of the colostrum. Pooled human im­munoglobulin is also used as a source of antibody in a number of infections (measles, small pox and se­rum hepatitis) during the period of incubation to modify or prevent the infection.

The Immune Response:

(A) Humoral Immune Response:

After first contact of the tissue with the antigen, there is an interval of about two weeks before anti­body can be found in the blood and, during this initial period, there is intense activity in the antibody form­ing tissues. After primary antigenic stimulation, there is a rapid increase in cell proliferation and in the synthe­sis of protein in cells of the lymphoid organ.

The antibody which can be detected during the “primary immune response” does not reach its peak. Then, after a day or two, there is a remarkable rise in the level of antibody which reaches its peak. This “secondary immune response” can be boosted to even higher levels by a further injection of antigen (booster or recall dose) until no further increase occurs and it slowly falls.

Once an animal has responded to a single dose of a live attenuated (e.g., small pox or polio vac­cine) antigen the animal retains a “memory” of the antigen so that months or years later it responds with a rapid mobilisation of anti-forming cells. Vaccination with even non-living agent such as tetanus or diph­theria toxoid, if given in two spaced doses provides, several years of useful protection against infection even though soon after vaccination the level of anti­bodies falls to a low level, the “memory” is retained.

To obtain a maximum response, the interval between the primary and secondary injection should not be too short and an interval of less than ten days is likely to reduce the level of the secondary response. This allows time for a maximal increase in the number of antibody forming cells which can be stimulated by subsequent injection.

Determinants of Acquired Specific Immunity:

1. Form:

The natural ability of antigen to in­duce an immune response may be enhanced by al­tering or mixing it with another substance known as adjuvant. The antigen can be absorbed on a mineral gel such as aluminium hydroxide or aluminium phos­phate, e.g., Alum precipitated antigen.

Such particulate form of antigen seems to be able to initiate antibody production much more effectively than the same antigen in non-particulate form. The effect is not yet fully understood, but it may be due to the direct ef­fect of the particulate antigen on the lymphoid cell membrane leading to more effective transformation of the cell for antibody formation than that can be brought about by antigen in solution.

Another method has been developed to en­hance antibody response, i.e., preparation of a water-in-oil emulsion. The emulsion forms a depot of anti­gen in the tissues from which small quantities of an­tigen are released continuously, sometimes for a year or more.

Example, Influenza Vaccine:

Route:

If an antigen is given intravenously, most of the antibody is produced by spleen, and some in lung and bone marrow; if given subcutaneously or intradermally, the antigen travels via lymphatic’s to local lymph nodes where antibody is produced ini­tially.

Dose:

The antibody response is proportionate to the dose of antigen. If the antigen dose is increased, the immune response is small, so if a particular level of antigen is increased then there will be specific paralysis of the antibody forming tissues – Immuno­logical tolerance.

(B) Cell Mediated Acquired Immune Response:

Cell mediated immunity occurs particularly in infections by microorganisms that enter and grow within the tissue cells such as viruses, tubercle bacilli, brucella and some salmonella. It seems likely that the cell mediated immune response is involved in pro­tection against infective agents, particularly those causing intracellular infection.

Cell mediated immunity is manifested by de­layed hypersensitivity around the site of infection. The cell mediated response is initiated in different areas of spleen and lymph node (white pulp of spleen and Para cortical areas of lymph).These areas are con­trolled by thymus—so these lymphocytes are called T-cells.

Very recently a number of yet uncharacterized non-antibody lymphocytic factors (non-specific anti­bodies) are described and are called as lymphokines. They are released by sensitized lymphocytes, T-cells, in contact with antigen.

Functions:

Lymphokinehas:

1. The property of inhibition of the in vitro migration of macrophages on glass slide.

2. Macrophages can be grown in culture in capillary tube and will normally migrate out of the open end of the tube into the culture fluid.

3. A factor released by lymphocytes exposed to antigen will prevent this migration by causing the macrophages to stick to­gether. This factor is likely to be released also in vivo and may be responsible for the accumulation for macrophages in cell mediated immune reaction.

4. Another factor released from sensitized lymphocytes after exposure to antigen causes stimulation and proliferation of normal un-sensitized lymphocytes. This is known as mitogenic factor.

5. Another lymphokine has a chemotactic effect on macrophages attracting them to the site of the inflammatory response.

6. A cytotoxic effect due to a lymphokine (lymphotoxin) has been demonstrated to damage the different types of cell.

7. A factor which increases capillary perme­ability (skin reactive factor) can also be shown to be released by sensitized lymphocytes.

8. It is generally thought that the lymphokines are the main effector mecha­nism in the cell mediated response.

Tissue Involved in Immune Response:

Lymph nodes, spleen and bone marrow are mostly engaged in the immune response, while lung and, to some extent, the liver, can take part in im­mune response. The specialised lymphoid organs (spleen and lymph nodes) take up foreign antigen and initiate the immune response.

The lymph nodes and spleen are made up of three types of cells which are involved in the initiation of the immune reaction. The three types of cells are lymphocytes, plasma cells and phagocytic cells of reticuloendothelial system. The lymphoid tissues responsible for humoral anti­body production are associated developmental with the gut and consist of lymphocytes and plasma cells of the lymph nodes and spleen.

In the humoral immune response, immunoglobulin’s are secreted into the blood by the lymphoid cells, predominantly plasma cells situated in the spleen and lymph nodes. In man, lymphoid tissue appears first in the thy­mus at about eight weeks’ gestation. Peyer’s patches are distinguishable by the fifth month and immu­noglobulin secreting cells appear in the spleen and lymph nodes at about 20 weeks.

From this period onwards both IgM and IgG globulins are synthesised by the foetus with IgM predominating. At birth the infant has a blood concentration of IgG comparable to, or sometimes higher than that of maternal serum having received IgG but not IgM via placenta from the mother.

The rate of synthesis of IgM in the infant increase rapidly within the first few days of life and does not reach adult levels until about a year. Cell mediated immune reactions can be stimulated at birth but these reactions may not be as powerful as in the adult.

Cells Concerned in the Antibody Production:

Cell cooperation between thymus dependent (T) lymphocytes and bone marrow derived (B) lymphocytes has been shown to occur. The (T) lymphocytes assist (B) lymphocytes in some as yet undefined way, possibly by concentrating antigen on their surface and presenting it to B cells or perhaps by releasing a lymphocyte activation product (lymphokine) to stimulate the B cells to respond to antigen.

In the spleen, the cells engaged in antibody pro­duction are found in the red pulp, whereas in lymph nodes, they are found in germinal centres of the cor­tex. Individual immunoglobulin producing cells synthesise the whole molecule of immunoglobulin both heavy and light polypeptide chains. Individual cells make only one class of immunoglobulin and the light chains are restricted to one of the two types (K and L). T and B lymphocytes are morphologically similar.

Initially, B cells produce IgM antibody and then switch over to IgG production. Antibody secreting “Russell body” is found in plasma cell. B cells are re­sponsible for immunoglobulin (Ig) synthesis and can either react to the direct contact with antigen or co­operate with T cells which are helper cells.

Protection and Immunity:

A particular microorganism may become patho­genic if it is capable to overcome the immune re­sponse system of the human host:

1. Human host can be protected from the bacterial infections by

(a) Antibody mediated immunity, or

(b) Cell mediated immunity.

(a) Antibody Mediated Immunity in Bacterial Infections. Because of the production of the exotoxin, some microorganisms (i.e., Corynebacterium diphtheriae, Clostridium tetani, Cl.welchii, Cl.botulinum) are patho­genic.

Antibodies acquired by immunization or previous infection or given pas­sively as antiserum are able to neutralize the bacterial toxins. Bacterial toxins are enzymatic in nature; most probably, the antibody interacts with the active site of the enzyme and neutralizes effectively the toxin.

When bacteria do not produce exotoxins, anti­bodies get attached to the surface of the bacteria. The most important effect of this attachment is to encourage phagocytosis by blood macrophages or polymorphs (opsonisation).

The mechanism is that the antibody alters the surface change of organisms and makes them more susceptible to phagocytosis. The phagocytes can also destroy the organisms by digesting by various en­zymes produced in the intracellular lysosomes; but Streptococcus pyogenes, typhoid bacillus, Mycobac­terium tuberculosis are able to resist the digestion by enzymes. If immunity exists due to previous infec­tion or artificial immunization, streptococcus is sus­ceptible to intracellular digestion, whilst the smooth strains are able to resist the digestion.

In case of enteric infections (typhoid and para­typhoid due to salmonella) antibodies are secreted into the intestinal lumen and attack these organisms before they invade the intestinal mucosa. These anti­bodies are known as “coproantibodies” and they are of IgA type which is selectively produced in mucous membrane of respiratory and intestinal tract.

Other effects of antibody attachment to the surface of the organism are:

(i) The lysis of Gram-negative bacteria brought about by the activation of complement system. The effect of the complement is to digest the cell wall of lipopolysaccharide so that the structural mucopeptide is exposed to the attack by lysozyme.

(ii) Attachment of bacteria to red blood cells in presence of antibody and complement. This phenomenon which is known as “Immune adher­ence,” may encourage the phagocy­tosis.

(iii) Antibody precipitated on the surface of certain parasitic nematodes can block the excretory orifices of the nematodes.

(b) Cell mediated Immunity in Bacterial In­fections. Macrophages from animal im­mune to tubercle bacilli are more actively phagocytic than those taken from a nor­mal animal. Similarly, macrophages from brucella infected animals showed en­hanced cellular immunity. The same was observed in Listeria monocytogenes.

2. Immunity in Viral infections:

(a) Antibody Mediated Immunity:

In virus in­fection, the antibody is mostly efficacious, if the virus passes through the blood stream to reach its target organ. For ex­ample, poliovirus initially crosses the in­testinal route, invades the blood stream and passes on rare occasions to the spinal cord and brain where it proliferates. Anti­body present in the blood stream can neutralize the virus before it reaches its target cells in the nervous system.

A number of virus behave in the same way (e.g., virus of measles, small pox, mumps, rubella and chicken pox). In case of Influenza and common cold, the viruses do not pass through the blood stream as their target organ is the respiratory mucous mem­brane. In this type of infection, a high level of anti­body will be less effective against these viruses in comparison with its effect on blood borne viruses.

Most of the antibody should pass through the mu­cous membrane into the respiratory secretion to act on such respiratory viruses. There is very little IgG and often no IgM in the content of mucous secretion, as this mucous membrane is not, most probably, very permeable to their clashes of antibody. But IgA is the predominant immunoglobulin in the mucous secre­tion.

IgA is manufactured by the plasma cells in the lamina propria of the mucous membrane and secreted in the nasal secretion. IgA has neutralizing activity against common cold viruses. Intranasal administra­tion of live attenuated Influenza virus can stimulate local production of IgA antibody in the mucous mem­brane of the nose.

The high degree of immunity was provided by live attenuated oral poliovirus may be due to locally produced antibody in the gut by neutralizing the vi­rus even before it reaches the blood stream. After oral vaccination, IgA against poliovirus has been dem­onstrated in the faeces, in duodenal fluid and in sa­liva, whereas no antibody was demonstrated after injection of inactivated poliovirus vaccine.

(b) Cell Mediated Immunity:

The children with congenital hypogammaglobulinaemia can recover from virus infection without pro­ducing any demonstrable virus neutralizing antibody, because of cell mediated im­munity. The patients with Swiss type agammaglobulinaemia and an additional cell mediated deficiency are very much sus­ceptible to virus infection and ultimately they may die.

3. Immunity to Protozoa and Helminths:

The life cycle of parasite is complicated and the immune response can be effective, if the life cycle is interrupted at a stage when the parasite is accessible to immune process. The antibody may, most prob­ably, have the access to the parasite if the perme­ability of the red blood cell membrane is altered so that the immunoglobulin can enter and attack the parasite. In protozoal infection, immunoglobulin pro­duction is increased and all classes of immunoglobulin’s are involved.

Life cycle of helminths is also complex as in protozoa. Protective immuno responses may act only at an early stage in the life cycle. IgE (reaginic) antibody with pulmonary eosinophilia will appear in the parasitic infections and the immediate hypersensitivity reactions of anaphylactic type (Type 1) is involved in the pathogenesis of helminth infections.

Allergy (Hypersensitivity):

Von Pirquet coined the term allergy (Gr. alios, altered, changed; ergon, action) to describe the al­tered reactivity of an animal after exposure to a for­eign antigen. The allergy included both immunity and hypersensitivity. It is now, restricted to refer only to hypersensitivity.

There are two forms of hypersensitivity reac­tions:

1. Immediate allergy, and

2. Delayed allergy.

1. The immediate form appears rapidly after exposure of a sensitized person to further dose of antigen and usually depends upon the liberation of histamine—pharmacologically active mediator sub­stance which is formed under the influence of anti­gen antibody combination.

2. The delayed form appears more slowly (usually after 24 hours) and depends upon the immunologically activated lymphoid cells. Various classifications of hypersensitivity reac­tions have been proposed, but the most widely ac­cepted one is that of Coombs and Gell. The classifica­tion of Coombs and Gell recognises four types of hy­persensitivity reactions, three of which come under the heading of immediate reaction.

Type I:

Anaphylactic reactions;

Type II:

Cytotoxic or cytolytic reactions;

Type III:

Toxic-complex syndrome;

Type IV:

Cell-mediated delayed hypersensitivity form of reaction.

Very recently, Type V, mixed type, mixed anti­body and cell mediated reaction has been recognised.

1. Anaphylactic Reactions (Ana Phylaxis) Type-I.

If a guinea pig is injected with a small dose of a foreign antigen (egg albumin), no adverse effects are ob­served. When a second injection of egg albumin is given intravenously after an interval often days, a condition known as “anaphylactic shock” develops — the guinea pig becomes restless, starts to chew and rubs the nose with its own front paws, its respira­tion is labored; the animal becomes cyanosed, may develop convulsions and, ultimately, die.

The initial injection of antigen is termed -“sensitizing dose” and the second dose is known as “shocking dose” During the interval between the two injec­tions, the animal has formed antibody which gets at­tached on the surface of cells; later, there will be in­teraction of shocking dose of antigen with the newly formed antibody on the surface of cells ultimately leading to “anaphylaxis”

In anaphylactic reaction, four pharmacologically active substances have been implicated — the most important is histamine. The release of histamine can be demonstrated in vitro by exposing antibody sensitized pieces of tissue to the contact of antigen.

(Sensitisation may be induced by prior injection of antigen into the animal supplying the tissue which makes antibody as described above or tissue may be sensitized passively by the addition of antibody pro­duced in other animal).The typical contractions in­duced in isolated strips of uterus and intestine are called Schultz — Dale reactions after the discover­ers.

In man, there are two types of reactions, sys­temic and local, which are related to the mode of entry of the shocking dose of antigen into the body. If the antigen is injected parenterally as in the case of serum (e.g., horseantitetanus serum), drug (penicil­lin) or perhaps by the bite of an insect, the systemic form of anaphylaxis is likely to develop, which is char­acterised by dyspnea with bronchospasm, sometimes skin rashes, a fall in blood pressure and occasional death.

On the other hand, if the antigen comes prima­rily in contact with the respiratory mucous membrane, then in a sensitized individual, the local form of ana­phylaxis may develop, i.e., hay fever, asthma. When the appropriate antigen (e.g., nuts, fish, strawberries) come in contact with the intestinal mucous mem­brane of a sensitized individual a mixed form of reac­tions may develop with the intestinal symptoms, urti­caria, skin rashes and sometimes symptoms of asthma.

Antibodies in Anaphylactic Reactions:

In man, reaginic antibody is responsible for the sensitisation of the tissues for anaphylactic reactions. This antibody has an affinity for tissues and can read­ily be detected in the serum of a sensitized individual by injecting a small quantity of this serum into the skin of a normal recipient, which is followed by intro­duction of the appropriate antigen into the same site of injection, after an interval of 24-48 hours. A wheal and flare erythematous reaction develops within about 20 minutes at the site of injection, just like a response to an injection of histamine. This reaction is known as Prausnitz-Kustner (PK) test according to its originators.

2. Cytotoxic or cytolytic reactions, Type II. The antibody directed against a cell which is associated with an antigen brings about cytotoxic or cytolytic effect which involves participation of comple­ment:

(a) In incompatible blood transfusion, the cy­tolytic effect of antibody on foreign red blood cells was observed.

(b) Haemolytic disease of new born has a simi­lar mechanism.

(c) Sedarmid purpura is a cytotoxic reaction which is due to the union of the drug sedarmid with platelets, as a result an an­tibody response against the platelet ab­sorbed drug brings about destruction of the platelets and thus cause purpura. Therefore, the drug sedarmid is withdrawn from the market.

A variety of infectious diseases due to salmo­nella and Mycobacteria are associated with haemo­lytic anaemia. Studies of salmonella infections re­vealed that the haemolysis is apparently due to an autoimmune reaction against a lipopolysaccharide endotoxin which becomes coated on the erythro­cytes. A detailed study has been conducted in Salmo­nella gallinarum infections in chickens.

3. Toxic complex syndrome, Type III. This reac­tion is due to the combination of antigen with circulating antibody leading to formation of micro precipitates in and around small blood vessels with consequent inflammation and subsequent me­chanical blockage of the vessels causing interference with the blood supply to surrounding tissues.

There are two types of reactions:

(a) Systemic form — Serum sickness;

(b) Local form — Arthus phenomenon.

Serum sickness develops in individuals given injection of foreign serum and it is described soon after the parenteral ad­ministration of horse anti-tetanus serum (ATS).Symptoms are asthma or laryngeal edema. The organs affected are kidney, heart and joints. Nowadays the most likely causes of serum sickness are drugs (peni­cillin) which form drug-protein complexes in the host.

Arthus Phenomenon:

Like serum sickness, Arthus phenomenon is brought about by the formation of antibody-antigen com­plexes, but in this case, the phenomenon is local at the site of injection. Arthus reac­tion occurs in the walls of small blood ves­sels in the presence of large quantities of IgG antibody, which forms micro-precipitates with antigen.

This reaction may occur in diabetic patients who have re­ceived many injections of insulin and have developed high levels of IgG antibody to antigenic constituents in the insulin prepa­ration. Similar reaction is also noted in case of anti-Rabies vaccination.

4. Delayed hypersensitivity, Type IV or Cell Me­diated hypersensitivity. This type of reaction occurs after 24-48 hours after injection of antigen. The clas­sical example of this type of reaction is tuberculin response which occurs, when an individual previously or currently infected with tubercle bacilli is given an intradermal injection of 0.1 ml of al: 1,000 dilution of a protein extract of tubercle bacilli (purified pro­tein derivative, PPD), 24-48 hours later an indurated inflammatory reaction of variable size can be seen in the skin. The injection site is infiltrated with large number of mononuclear cells — lymphocytes. The mechanism is not yet well understood.

Macrophages in delayed hypersensitivity reac­tion sites may contribute lysosomal enzymes from their intracellular granules and thus cause further in­flammatory changes. When macrophages come in contact with anti­gen, the sensitized lymphocytes release a number of non-antibody factors (i.e., lymphokines).

Which are:

(a) A factor inhibiting macrophages migration;

(b) A chemotactic factor; and

(c) A cytotoxic factor. The role of these sub­stances in delayed hypersensitivity is not clear.

Delayed hypersensitivity reactions are observed in brucella, salmonella, mycobacterium, pathogenic fungi and a wide range of virus (herpes simplex, mea­sles, small pox, vaccinia) infections. It can develop after sensitisation to a variety of metal (nickel and chromium) and dyes (hair dyes). Skin sensitisation to penicillin is common after topical application of the antibiotic ointment or cream — clinical signs are redness, swelling, vesicles scaling and exudation of fluid.

Types of Allergy:

Whether immediate or delayed allergy, the ba­sic explanation of allergic manifestation is that a re­action between antigen and its specific antibody has occurred in the body. If this reaction occurs in the blood, it will not cause allergic response and is the normal defensive process. If antigen-antibody com­bination occurs in or on the tissue cells, there is al­lergy.

There are two types of allergy:

1. Immediate allergy is due to precipitin reac­tion occurring in or on certain tissue cells. It is charac­terised by the painful itching wheals known as “hives” This type of allergy may occur after eating certain foods or after injections of therapeutic serum; other manifestations are: intense gastroenteritis appear­ing within few minutes after consuming certain foods to which the victim is allergic and asthma after con­tact with pollen or with certain animals.

Shock Tissue:

The signs, symptoms and appear­ance of immediate allergic reactions depend upon the tissue cells which suffer precipitin reaction. The tissues that suffer are known as “shock tissues” If the tissues are smooth muscle fibres, the muscle contracts.

If the muscle fibres of gastrointestinal tract contract, diarrhoea and cramps may result; if the gravid uterine muscle contracts, there is abortion; asthma may occur, if the bronchioles contract. Besides smooth muscle contraction, edema and inflammation of mucosa are manifested in the allergy. 0.5 to 1 ml. of 1:1,000 epinephrine quickly reduces the swellings. Anaphylaxis is one of the most dramatic mani­festations of the immediate type of allergy.

2. In delayed allergy, the tissue response ap­pears after 24 hours.

The tuberculin reaction is a best example of delayed allergy.

Histamine in Allergy:

In immediate allergy, an initial substance called histamine is released by tis­sue cells. The histamine causes many of the symp­toms seen in hay fever, bee stings, serum reactions, antibiotic reactions and common colds. These discom­forts can be relieved by antihistaminic which cause contractions of blood vessels.

Some Common Allergies:

Pollen Allergy:

The pollen (sperm cells) of many plants — grasses, roses, ragweed — may cause al­lergy in some persons. When the proteins of the pol­len come in contact with nasal and respiratory mu­cous membrane of the individuals, they are absorbed locally and set up a hypersensitivity condition of these tissues.

When the individual inhales later more of the pollen proteins, the nose and throat become irritated, swollen and edematous. Because of the severity of the reactions against pollen, there develop a symp­tom of severe “cold” which is commonly known as “hay fever” or nose fever. These conditions may be relieved by antihistamines and nasal decongestants.

Food Allergy:

A person may become sensitized to certain foods (strawberries or cod fish). Due to a slight gastrointestinal irritation, lesion or disturbance, some of the food is absorbed undigested through the stomach or intestinal wall into the blood, the re­sult is the same as if the protein has been injected.

The next time or perhaps every time, the same per­son eats strawberries or cod fish, he will react very violently. There will be allergic manifestation in the form of rash or blotches on the skin which itch persistently. These conditions are known as “hives”. Some other victims may exhibit nausea, vomiting, gas­trointestinal irritation and general symptoms. Antihistaminic are useful in these conditions.

Infection Allergy:

Due to microbial proteins en­tering into the blood stream, certain cells of the body may become sensitized to these microbial proteins resulting into the delayed allergy. A person suffering from tuberculosis may become allergic to tuberculosis. The relapses in rheumatic fever are due to repeated allergic responses (with damage to heart muscles) to reinfection with certain streptococci to which the patient is already allergic. Allergy may also develop in viral, fungal, worm infections, such allergy is called as Infection allergy. Most of the damaging effect of infectious diseases is due to infection allergy.

Serum Reactions:

There are two forms of serum reactions: one is acute and serious form resembling anaphylaxis characterised by collapse, dyspnea, oc­curring after a few minutes after the injection of thera­peutic serum (e.g., diphtheria antitoxin); the other one is milder, delayed form known as serum sickness.

Both forms depend upon the precipitin in the pa­tient. Either type of serum reaction is due to animal (horse) protein in the therapeutic serum, not to the specific antitoxin. Symptoms are chill, nausea, exten­sive painful itching hives or serum rash. Pain in the joints may occur later. If the patient is serious, there may be fatal end.

Delayed Serum Reactions:

Serum sickness may appear after ten to fifteen days after a therapeutic use of horse serum. It is assumed that, during this period, the precipitin to horse serum protein may accumulate in the tissues. After reaching a certain concentration, the precipitins react with the proteins (even in residual portion) of the therapeutic horse serum which still remain in the body.

If the therapeu­tic horse serum is to be given parenterally; a prelimi­nary skin test for hypersensitiveness to horse serum may be performed as described below. If the test is positive, the patient should be desensitized by giving a number of a very small (0.1 to 1 ml.) doses of the serum at intervals of an hour or so before the main injection. The possible shock to the patient is lessened. This process is called as “desensitisation”

Persons with previous injections of serum usu­ally require desensitisation. Sometimes, even in the persons who have not had the previous injections of serum, the serum sickness may also occur.

Atopy:

Some individuals inherit not allergy it­self, but a certain increased tendency to form reagins and thereby to develop allergic reactions (i.e., asthma, hay fever) and perhaps certain forms of arthritis and rheumatic heart disease. These persons are called at­opic (and are called unfortunate).

Drug Idiosyncrasies:

Certain plant perfumes, cos­metics, drugs, rubber goods, dyes etc. may also cause immediate allergy like responses (hives, dermatitis, asthma) which may be basically different from true allergy and which may not be previously dependent on specific antibodies at all. These allergy like reac­tions restricted to only to certain individuals are known as drug idiosyncrasies. The mechanism is not yet well understood.

Binding Power of Tissues:

Because of natural infections or artificial injec­tion of certain antigens (bacterial cells and exotoxins), cutaneous and mucosal tissues often acquire the property of combining with and holding fast to these specific antigens entering the body. This is a defence mechanism and is probably one manifestation of al­lergy.

These tissues are said to be sensitized to the antigens. Once sensitized, the tissues tend to hold on and bind to these specific antigens, infections or tox­ins to prevent their further spread from the site of localisation whenever, they gain entry into the body. The tissues may be damaged locally, but generalized infections or intoxication is prevented.

The binding power of the tissues is the basis for the beneficial effect of several methods of artificial immunization as per example, Bacillus Calmette Guerin (BCG) vaccine. This is also of great importance in resistance to brucellosis, syphilis and other chronic diseases.

Skin Tests for Hypersensitiveness:

When a very small dose of sensitizing dose is injected or scratched into the skin of a person or ani­mal hypersensitive to that particular antigen, a wheal (a large, red, swollen area) will develop after an inter­val of time. This allergic reaction persists for an hour or more. Hypersensitiveness to horse serum is tested in this way by injecting a very small quantity of the serum to be used therapeutically.

Similarly, for testing food or pollen allergy, alco­holic or aqueous extracts of various foods or pollens, likely to be involved, are introduced into (not under) the skin either by scarification or injection and the results are noted.

Application to Nursing:

The intelligent professional nurse, who acquired a sound knowledge about allergy during her training, will be able to understand and handle the untoward reactions encountered when certain sensitized indi­viduals come into contact with that particular antigenic (allergic) substances — sera, antibiotics, some drugs, vaccines, some foods. A severe allergic reaction may ensue following the injection of an antigenic sub­stance. This reaction can be prevented and the pa­tient’s life can be saved by the perceptive nurse.

For example, since the patient is under constant obser­vation of the nurse on duty, the patient may narrate that he is allergic to eggs (foreign protein); if the phy­sician, unaware of this allergic reaction, prescribes a viral vaccine prepared from chick embryo, the nurse should inform immediately the physician about the allergy of the patient to the egg, so that a severe, possible fatal allergic reaction can be averted.

If the patient who had a previous injection of horse serum receives again a dose of antitoxic serum prepared in horse, may have a severe and even fatal reaction to the serum, which, although given to provide the pa­tient with immediate protection against a specific disease, becomes a very highly fatal dose to the pa­tient, since the patient is allergic to horse serum.

It is quite essential to understand that the pa­tient reaction to the antitoxic serum is not due to antibodies present in it; but due to animal (horse) proteins in the serum. Thus, the proteins may react to any horse serum irrespective of presence of antibodies.

The well informed and trained nurse should keep in readiness epinephrine and antihistaminic to avoid such a fatal allergy which may occur due to the parenteral administration of horse serum for passive immunity and she should always be careful about immediate and delayed allergy to an allergenic sub­stance and advise the patient to avoid in future the injections of this particular allergic substance.

Autoimmunity:

Immune response of an animal does not, under normal circumstances, react against its own body constituents. It is a fundamental characteristic. How­ever, there is a mechanism that makes the cells of the immune system to recognise what is “foreign” and what is “self”.

Ehrlich and Morgenroth (1900) after immunizing a goat with red blood cells of other goats found that the animal readily made antibody against the red blood cells of other goats, but this antibody failed to react with the animal’s own red blood cells. Thus it was understood that the immunological response to self antigen was prevented.

Coombs et al (1946) developed a technique to detect the presence of antibody globulinonred blood cells by adding an anti-globulin serum which, after reacting with the globulin coating the cells, linked the cells together and agglutinated them. This tech­nique is known as “Coombs” or “anti-globulin” test and could detect a variety of human and animal diseases in which antibody capable of reacting with a number of different antigens of the individual own tissue.

Haemolytic anaemia in man, in which red blood cells are coated with antibody globulin, was first dis­covered with Coombs’ test. Systemic lupus erythematosisi SLE) in which antinudear factor (ANF), — an antibody to nucleus of the cell, is involved. This autoantibody serum factor is present in the blood of patients suffering from connective tissue disease. This (SLE) disease was the second discovery by adopting

Coombs’ technique:

Interstitial thyroiditis, (known as Hashimoto’s disease) in which an antibody to thyroglobulin is involved, was the third discovery.

The control mechanism for autoimmunity tol­erance could be overcome by two ways:

1. The first category is the evasion circumven­tion of a normal functioning mechanism, and

2. The second one is the failure of the mecha­nism itself.

In the first category, evasion may occur in two ways:

(a) a particular body antigen is not normally accessible to the cells of the immune sys­tem as the antigen is hidden within a cell or tissue; in the second category, a tissue antigen is altered in some way by a chemi­cal, a drug, or an infective agent. The sperm antigen and lens antigen of the eye are sequestered (hidden) antigens; whereas antigen in the human colon is altered an­tigen.

1. In the human colon, there is an antigen which can be extracted. This antigen is similar to a polysac­charide antigen present in Escherichia coli O₁. So, it is believed that the inflammatory conditions of the co­lon known as “ulcerative colitis” in which anti-colon antibodies are involved and produced as a result of the immune response initiated by the cross-reacting bacterial antigen.

2. Group A streptococci causing rheumatic fe­ver have an antigen common with that of human heart. Antibody to the antigen of streptococci may cross-react with the antigen of human heart causing heart lesions.

3. Streptococci carry surface antigen similar to that found in human glomeruli and so streptococcal infection is associated with the development of acute glomerulonephritis.

4. The association of Mycoplasma pneumonia with primary atypical pneumonia may cause the de­velopment of IgM agglutinins which, in the cold, re­act with the patient’s own red cells and may produce haemolytic anaemia.

Studies on the effect of herpes or rabies viruses on tissue culture cells suggested that the viruses are able to bring about alterations of cell membrane. The association of virus with lymphocyte membrane may possibly interfere with normal lymphocyte reaction to antigen.

Pathogenesis of Autoimmune Diseases:

Three types of hypersensitivity reactions are involved in autoimmune diseases:

(i) Cytotoxic reaction. Autoimmune haemolytic anaemia is an example of the cyto­toxic reaction in which two types of anti­bodies are involved.

1. IgM antibodies which agglutinate the patient’s own red cells in the cold.

2. IgG type antibodies, which do not cause direct agglutination of the red cells, can be detected by Coombs’ test with anti-globulin serum.

In mycoplasma infections, the IgM cold agglutinins react with a common red cell antigen known as the antigen. 

(ii) Toxic complex type reaction. Complex of antibody and tissue antigens, particularly nuclear antigens are the cause of glomerulonephritis and systemic lupus erythematosis (SLE) where a wide variety of anti-tissue antibodies is found.

(iii) Cell mediated reaction. In both man and animal the development of a number of autoimmune states is paralleled by the ex­istence of cell mediated reaction of the delayed hypersensitivity type demon­strated by skin test with tissue antigens. There is an association between the reactions of cell mediated immunity and humoral antibody in induced autoimmune orchitis in guinea pigs.

Other disease associated with autoimmune states is rheumatoid arthritis in which IgM antibody called rheumatoid factor is present in the patient serum. This factor can be detected in vitro by its ability to agglutinate red blood cells or latex particles coated with IgG globulin.