Non specific resistance is the defense of our body from any kinds of the pathogens. It includes skin and mucous membrane, phagocytosis, inflammation, fever, production of antimicrobial substances.
Contents
1. Skin and Mucus Membranes:
Skin and mucus membranes provide the first step of defense to the body against invasion of the pathogen.
It acts both as mechanical barrier as well as chemical factors:
i. Mechanical Factors:
Skin acts as an outer barrier of keratinized epithelium to microorganisms, chemicals and non-living agents. It consists of over 15% of dry weight of the body. It contains two portions, the dermis (inner and thicker portion of skin) and epidermis (the outer thinner portion influenced by the external environment).
Epidermis comprises of tightly packed layers of epithelial cells. The upper layer of epithelial cells is dead. It protects the inner tissues. As a result of cuts, burns, wounds, etc. infection of skin and underlying tissues frequently occurs. When the skin frequently remains moist, the chances for skin infection by fungal pathogens get increased.
Mucus membranes lack the thickened layer but have the other features that provide defense. They line the gastrointestinal, respiratory, urinary and reproductive tract. The epithelial layer of mucus membrane secrets mucus which is a free moving liquid produced by globlet cells. It consists of inorganic salts, many organic molecules, loose epithelial cells and leucocytes.
Mucous prevents the tract from dessication. Some pathogens e.g., Treponema pallidum, Mycobacterium tuberculosis, Streptococcus pneumoniae, etc. attached to mucus (if are in sufficient number) can penetrate the membrane. Mucus offers less protection than the skin.
(a) Lachrymal Apparatus:
Lachrymal apparatus is found in eyes and also associated with defense against eye infection. It forms and drains away the tears. Lachrymal gland is present towards the upper and outermost part of both the eye socket. This gland produces tears which is spread over the surface of eye ball through blinking.
Continuous washing action protects the eyes from setting on eye surface. Whenever microorganisms come in contact of surface of eyes the lacrimal glands start secreting tears heavily and either dilute or wash away the microorganisms or irritating substances on eye surface.
(b) The Other Glands:
Salivary glands produce saliva that also wash microorganisms from teeth and mucus membrane of mouth. Similarly cleansing of urethra and vagina by the flow of urine and vaginal secretion, respectively also wash microorganisms from the respective sites and provide some sorts of defense.
The mucus membrane of nose possesses mucus coated hairs that filter the air after inhaling, and trap microorganisms, dust, etc. However, the cells of mucus membrane of the lower respiratory tract are covered with dust and microorganisms which have been trapped towards the throat. This so called ciliary escalator keeps the mucus blanket moving towards the throat at the rate of 1-3 cm/h. After coughing or sneezing the escalators speed up.
ii. Chemical Factors:
There are certain chemical factors of skin and mucus membrane that play roles in providing defense such as gastric juice, enzymes, sebum, etc. Sebaceous (oil) glands of the skin produce oily substance which is known as sebum.
Sebum prevents the hair desiccation and becoming brittle, and form a protective film over skin. Sebum contains unsaturated fatty acids and to some extent acetic acid. Sebum inhibits the growth of microorganisms. This secretion lowers down the pH between 3 and 5, and arrest the growth of many microorganisms.
Skin also possesses sweat glands that produce perspiration. Perspiration removes the wastes and wash microorganisms from skin surface and maintains body temperature. Perspiration contains the enzyme lysozyme that dissolves cell wall of Gram-positive and a few Gram-negative bacteria. The other sources of lysozyme are saliva, mucus, tears, nasal secretion and tissue fluids.
Gastric juice is secreted by the glands of stomach. It contains HCl, digestive enzymes and a little amount of mucus. Very low pH (1.2) i.e. high acidity of gastric juice of stomach kills the bacteria and bacterial toxins. However, the enteric pathogens are protected by the food particles and, therefore, enter the intestine through the gastrointestinal tract.
2. Phagocytosis:
Phagocytosis (means eat and cell) refers to ingestion of microorganisms or any particulate material by a cell. It is also a method of nutrition of some protozoa such as Amoeba, but the mechanism discussed here is related to the defense mechanism of body provided by white blood cells through phagocytosis. Before discussing the mechanism of phagocytosis we should learn the components of our blood.
Blood consists of fluid known as plasma which contains different constituents such as erythrocytes or red blood cells (RBC), leukocytes or white blood cells (WBC) and thrombocytes or platelets (Table 22.2).
The leukocytes can be divided, on the basis of granules in their cytoplasm, into granulocytes and agranulocytes. Granulocytes contain three types of blood cells (e.g. neutrophils, basophils and eosinophils) and agranulocytes contain two types of cells (lymphocytes and monocytes) (Fig. 22.1).
The granules of cytoplasm can be observed under the microscope. After staining these granules take different stains with a mixture of acidic (eosin) and basic (methylene blue) dyes the granules of neutrophils take red and blue stains respectively. Granules or basophils stain blue with methylene blue, and that of eosinophils stain red with eosin.
Neutrophils can enter an infected tissue and kill microorganisms and foreign particle. The basophils can release substances like heparin (an anticoagulant) and histamine (in inflammation and allergic responses). Eosinophils are the phagocytes. After microbial infection or hypersensitivity their number increases. In agranulocytes the granules are absent. These are of two types, lymphocytes and monocytes.
The lymphocytes are of two types, B-lymphocytes and T-lymphocytes. The B-lymphocytes derived its name from its site of maturation in the bursa of fabricius in birds. The name turned out to be apt for its major site of maturation in mammals in bone marrow. The B-lymphocytes depend on the activity of bursa tissues, whereas the T lymphocytes (derive its name from thymus) depend on the thymus for their activity.
Thymus contains T cells but not B cells; similarly bone marrow consists of only B cells but not T-cells. Both the lymphocytes occur in lymphoid tissues (e.g. tonsils, lymph nodes, spleen, thymus gland, thoracic duct, bone marrow, and appendix, lymph nodes in respiratory-gastrointestinal and reproductive tracts).
The T- and B- cells cooperate in the presence of a third cell, Mechnikov macrophage. These provide immunity. Monocytes mature into macrophages and act as phagocytes. The leukocytes are derived from stem cells in bone marrow and enter the lymph system (lymph node, spleen, thymus, etc.).
i. Types of Phagocytes:
The phagocytes are of two types, granulocytes (microphages) and monocytes (macrophages). When a microbe infects granulocytes (neutrophils), monocytes move to the infected area. During migration, monocytes enlarge in size and called macrophages. Since these macrophages are migratory, they are also termed as wandering macrophages.
Some macrophages remain at a fixed position e.g. in the liver (Kupffer’s cells), lungs (aleolar macrophages), nervous system (microglial cells), branchial tissue, bone-marrow, spleen, and lymph nodes and peritoneal cavity surrounding abdominal organs. These macrophages are called fixed macrophages which constitute the mononuclear phagocytic system.
ii. Mechanism of Phagocytosis:
After infection the number of WBC increases in blood during the initial phase of infection.
At this stage they are phagocytic in nature. As the infection progresses the number of monocytes increases. They phagocytize the remaining dead or living microbial cells. When blood and lymph containing microbial cells pass through the organs with fixed macrophages, the cells of mononuclear phagocytic system kill them through phagocytosis.
The mechanism of phagocytosis can be divided into the following four steps (Fig. 22.2):
(i) Chemo-Taxis:
It is a phenomenon of chemical attraction of phagocytes to microorganisms. The chemotactic chemicals which attract the phagocytes are the components of WBC, and damaged cells, peptides derived from complements and microbial products.
(ii) Attachment:
The plasma membrane of phagocyte gets attached to the surface of a microbe or foreign material (Fig. 22.2 A). When there is a large capsule M protein attachment can be hampered. For example M protein of Streptococcus pyogenes inhibits the attachment of a phagocyte to their site. Similarly, Klebsiella pneumoniae and Streptococcus possess a large capsule and get escaped.
However, the large sized microorganisms or foreign material is trapped in blood clots, blood vessels or fibres of connective tissues. If the cell wall of microorganisms is coated with certain plasma protein promoting the attachment of microbe to phagocytes, only then they can be phagocytized. The coat proteins are called opsonins and the process of coating of plasma protein is known as opsonization.
(iii) Ingestion:
After attachment the plasma membrane of phagocyte extends short projections known as pseudopods which engulf the microorganisms or foreign materials. This process is known as ingestion (Fig. 22.2B). The extension of pseudopods continues until they contact and fuse, and surround the microorganism inside a sac which is known as phagocytic vacuole or phagosome (C).
(iv) Digestion:
After engulfment phagosome comes in the contact of lysosome that contains digestive enzymes and bactericidal chemicals (C). After making contact the membrane of phagosome and lysosome gets fused (D) and a single layered large structure is formed which is called phagolysosome (E). Within 10-30 minutes the contents of phagolysosomes degrade the microorganisms or foreign materials.
Lysosomes also contain lysozyme that breaks the peptidoglycan of bacterial cell wall. Lysozyme is more active at pH 4 which is an optimum pH of phagolysosomes because of production of lactic acid by phagocytes.
In addition, lysozyme also contains myeloperoxidase which binds with chloride ions to viruses and bacteria and finally kills them. After complete digestion of the foreign material the phagolysosome migrates towards the boundary of membrane and discharges the wastes (F).
Interestingly, toxin producing streptococci can kill the phagocytes and Mycobacterium tuberculosis can multiply within the phagolysosome itself and des-integrate the phagocytes. Also, the causal organism of brucellosis can remain dormant for several months or years inside the phagocytes. At this situation the role of immunity becomes vital.
3. Inflammation:
As a result of damage of body tissues, inflammation in the surrounding areas occurs. However, there are several causes of tissue damage such as physical agents {i.e. heat, radiant energy, electricity or sharp objects) infection of pathogens, chemicals (acids, bases, gases), etc. Therefore, mainly four symptoms characterize inflammation viz., swelling, pain, redness and heat.
For our system the inflammatory responses are beneficial and have the following functions:
(i) Inflammation possibly destroys the harmful agents and removes them or their by-products from the infected site.
(ii) If the harmful agents are not destroyed, it wards off the injurious agents and its bye products.
(iii) It repairs or replaces the tissues damaged by the injurious agents or their bye-products. Inflammation process occurs in the three stages, vasodilation and increased permeability of blood vessels, phagocyte migration, and repair.
i. Vasodilation and Increased Permeability of Blood Vessels:
After the damage of tissue, blood vessel is dilated where damage has occurred. Permeability of blood vessels also increases. As a result of vasodilation (i.e. increase in diameter of blood vessels) flow of blood to damaged area is increased. This is the reason why damaged area turns into red, and inflammation is induced due to heat.
Vasodilation is caused by histamine, a chemical released from the damaged tissue due to injury. In blood plasma another group of chemical (kinin) is present which too causes vasodilation. Collier (1962) has discussed the role of chemical mediators (kinin) in inflammation.
Kinin after being activated attracts neutrophils to injured area. From the damaged cells a substance, prostaglandin is secreted which is also associated with vasodilation. Due to increase in permeability of blood vessels, the clotting factors are delivered to the injured area where blood clots prevent the growth of microorganisms. This results in formation of pus in a localised spot.
ii. Phagocyte Migration:
Bretscher (1987) has given a comprehensive account of movement of phagocytes. Within an hour of inflammation, phagocytes (neutrophils and monocytes) appear and begin to stick on the inner surface of lining (endothelium) of blood vessel as the flow of blood gradually starts decreasing.
The process of sticking of phagocytes is known as margination. Thereafter, the second phenomenon, diapedesis occurs within two minutes. Diapedesis is a process of sneezing of phagocytes between the endothelial cells of blood vessels and reaching to damaged area.
Attraction of neutrophils occurs through chemotactic substances such as kinins, the components of complement system and secondary metabolites of microorganisms. Production and release of granulocytes from bone marrow ensures the steady stream of neutrophils.
Monocytes follow the granulocytes into the infected area as inflammation continues. In the early stage of infection granulocytes predominate but they are short lived. When monocytes are produced in tissue they undergo changes and become wandering macrophages which predominate during later stages.
They are several times larger and potential enough to phagocytize the damaged tissue, destroyed granulocytes and infectious microorganisms. After phagocytosis, the granulocytes or macrophages themselves die. After a few days the area contains dead phagocytes, damaged tissue and fluid which collectively are known as pus. Pus formation subsides later on, and gradually destroyed after a few days.
iii. Repair:
Repair is a process through which the tissue replaces the dead cells at the end of inflammation. During the active phase of inflammation repair starts but completes after removal of dead or damaged cell, the ability of which depends on the tissues involved.
For example, skin has a high capacity for regeneration, whereas nervous tissues in the brain and spinal cord do not regenerate at all. When the stroma (supporting connective tissue) or parenchyma (functioning part of tissues) produces new cells, the damaged tissue is repaired.
4. Fever:
An abnormally high body temperature is known as fever which is caused by bacterial or viral infection or bacterial toxins. It is obvious that hypothalamus (a part of brain) controls the body temperature and, therefore, sometimes it is called the body’s thermostat as it sets the temperature normally at 37°C (98.6°F). When antigens affect hypothalamus, body’s temperature goes up.
For example, when phagocytes ingest the Gram-negative bacteria, the lipopolysaccharide of bacterial cell wall i.e. endotoxin is released that induces the phagocyte also to release interleukin-1 (endogenous pyrogens). Interleukin-1 helps the production of T-lymphocytes. In turn, interleukin induces-hypothalamus to produce prostaglandins that result the hypothalamus to a higher temperature that causes fever.
Fever perists for a long duration until bacterial endotoxin or interleukin-1 is released. At high temperature the body responds with constriction of blood vessels, increased rate of metabolism and shivering (chilling). Chilling disappears after body’s temperature has reached the setting of thermostat.
Until the endotoxins are not completely removed, body’s temperature remains high. Thereafter, it is maintained normally at 37°C. In addition, fever inhibits the growth of some microorganisms in body. At high temperature body’s tissue repairs quickly, and the effect of interferon is intensified.
5. Antimicrobial Substances:
Some antimicrobial substances {e.g. proteins of the complement, and properdin systems and interferon) are also produced by the body after microbial infection.
Complements and Properdin:
In classical antigen-antibody complex, certain blood proteins also get associated and complement the immune response. These serum proteins are known as complements.
Similarly three serum proteins (e.g. properdin itself, factor B and factor D) which are commonly known as properdin, play a role in alternate pathway. Both types of proteins are related to the defense system. Properdin system is composed of the above three serum proteins which altogether constitute a high proportion of serum protein.
Therefore, about 20% different types of proteins (i.e. complements) which are found in normal blood serum. These are designated as C1, C2, C3, etc. Complements are very important to both nonspecific and specific defense against the microbial infection. Proteins of complement and properdin systems act in ordered sequence or cascade. The classical pathway initiates when the antibodies bind with antigens (bacteria or other microbial cells).
After a pair of antibodies recognise and bind to antigens, the C1 protein (which consists of 3 protein subunits) binds to antibodies and activated (Fig. 22.3). In turn C1 acts as an enzyme, activates C2 and C4 and splits C2 and C4 proteins (C2 into C2a and C2b, and C4 into C4a and C4b. C2a and C2b combines to form another enzyme that splits C3 into C3a and C3b.
The antibodies are not involved in the initiation of the alternate pathway, but interactions between protein properdin system and certain polysaccharides initiate this pathway. The polysaccharides are found on the cell wall of most of the bacteria, fungi and foreign RBCs of mammals. Properdin pathway interacts with Gram-negative bacteria, the cell wall of which contains lipopolysaccharide that releases lipid A (an endotoxin) and trigger the alternate pathway.
C3 is cleaved both by classical and alternate pathways into C3a and C3b; C3a is an active fragment. These fragments induce the three processes, cytolysis, inflammation and opsonization.
(i) Cytolysis:
It is a process of leaking of cellular contents of foreign cells through breaking their plasma membrane by the complements. C3b initiates a series of reactions involving C5, C6, C7, C8 and C9 which is collectively known as the membrane attack complex (MAC) (Fig. 22.3).
The activated proteins attack the microbial cell membrane and form a circular trans membrane channels (lesions). Through these lesions loss of ions and cytolysis occur. Use of the complements in this process is known as complement fixation which laid a basis for clinical test.
(ii) Inflammation:
The cleavage products, C3b and C5b, bind with mast cells (basophils) and blood platelets to trigger the release of histamine. Histamine elevates the permeability of blood. C5a fragment functions as a chemotactic factor which attracts phagocytes to the site of complement activation.
(iii) Opsonization:
Opsonization is a phenomenon of adsorption of certain antibodies or complement (C3 – C5 complex) specially C3b onto the surface of foreign material that results in stimulation in phagocytosis (Fig. 22.3). Opsonization is also known as immune adherence. Opsomzation is also one of the main antigen- antibody reactions associated with humoral antibodies. The two main opsonins (complement and certain antibodies) stimulate phagocytosis.
The complement stimulates T-cells to process for cell mediated immunity and release histamine from leukocytes, which in turn increases the capillary permeability and smooth muscle contraction. In general the local inflammation is caused due to these reactions. In contrast, another system (properdin system) also activates C3-C5 complex and initiates the protective responses.
The complement and properdin systems are very important in non-specific defense. The deficiency of C1, C2 and C4 causes collagen vascular disorder, consequently there develops hypersensitivity. C3 deficiency increases susceptibility to bacterial invasion and C5 deficiency (through C9) causes susceptibility to infection of Neisseria meningitidis and N. gonorrhoeae.
6. Interferon:
Viruses totally depend on their host cells for multiplication. However, during the course of multiplication the host cells may or may not be damaged. It is very difficult to check the virus multiplication without affecting the host cells. Interferons (IFN) are such class of antiviral proteins produced by certain animal cells after stimulation. Now-a-days interferons are used in causing immunity.
Interferons are host specific but not virus-specific. It means that interferons produced by human cells will show antiviral activity only in humans but not in another mammals. In contrast four interferons produced against a virus will also act against a number of other viruses. Even in humans different types of cells produce different interferons.
Human interferons are of the following three types:
(i) Alfa interferon (α-IFN or leucocyte IFN)
(ii) Beta interferon (β-IFN or fibroblast IFN), and
(iii) Gamma interferon (y-IFN or immune IFN)
With each principal group there are various subtypes of interferons. In humans interferon is produced by fibroblast in connective tissues, lymphocytes and other leukocytes.
The virus infected cells produce interferons in very low quantity which is diffused towards uninfected neighbouring cells. It reacts with plasma or nuclear membrane receptor and induces healthy cells to produce mRNA for the synthesis of the antiviral proteins.
These proteins act as enzyme and disrupt translation of viral mRNA, polypeptide chain elongation, etc. Since interferon is in low quantity, it does not badly affect the host cells. Its effect remains only for a very short duration. Interferons do not have any effect on viral multiplication in cells already infected.
Owing to its importance much emphasis is being laid on artificial production of interferon. For the first time clinical trial of interferon was done in 1981 to determine its anticancer effects. In recent years several companies have applied the recombinant DNA technology to produce interferon in certain bacteria.