In this article we will discuss about Immunity:- 1. Definition of Immunity 2. Determinants of Natural Immunity 3. Mechanism.

Definition of 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’s resistance power to infections.

Immunity:

(1) Natural (innate),

(2) Acquired.

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, Gonorrhoea, Typhoid, Rabies and Brucellosis affect both man 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 responses.

(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 vaccines virus and certain neurotrophic viruses e.g. polio virus.

The same condition may be observed in case of malaria. When the nutrition level is lowered, there may be deficiency of para-amino benzoic acid essential for the multiplication of malarial parasites.

Mechanism of 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 non­pathogenic 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 it 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:

1. 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.

2. 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 into the blood stream or tissue fluids, they are engulfed rapidly by various circulating and tissue fixed phagocytic cells.

These cells are of two types:

(i) Polymorphonuclear leucocytes or microphages of the blood;

(ii) 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 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, ultraviolet rays).

(d) Living agents (microorganisms, worms).

Four cardinal signs of inflammation are:

i. Rubor (redness, hyperaemia);

ii. Tumor (swelling);

iii. Calor (heat); and

iv. Dolor (pain).

They can be observed in a boil or in an inflamed area. The increased amount of blood in the dilated local blood vessels may cause the redness; the comparison 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 oedema—inflammatory exudation of fluid).

Hyperaemia:

It is due to increased amount of blood in an inflamed 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 gonorrhoea, 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 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 continue 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 mechanisms, tissue cells (fibroblasts) and blood vessels start to grow into the area of healing from periphery and form new healthy tissues.

This early healing process is known as Organisation. The tissue thus formed is a solid and 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.

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