Essay on the Types of Defence Mechanism | Human Health | Biology

Here is an essay on the ‘Types of Defence Mechanism’ for class 8, 9, 10, 11 and 12.

Essay # 1. Non-Specific Defence Mechanism:

It is similar for most of infections. It checks the entry of pathogens with the help of certain physical barriers or destroys the microbes through inflammatory response.

It includes two types of defence:

(A) First Line of Defence or Non-Specific External Defence:

It is formed of mechanical barriers (the sheathings that prevent the access of microbes to inside the access of microbes to inside the body, e.g., skin sweat, sebum etc.); lysozyme containing secretions and certain chemical compounds of blood which destroy the foreign organisms of their toxins.

(a) Physical Barriers.

Skin forms the non-specific external defence. The outer layer of epidermis is a horny layer called stratum corneum, and is formed of dead and flat cells. It is a germ-proof layer and acts as a physical barrier to the entry of bacteria and viruses

Sebum (Oil) and Sweat:

Secreted by sebaceous (oil) and soporiferous (sweat) glands of skin respectively, make the skin acidic (3 to 5 pH) due to presence of lactic acids and fatty acids. Some useful bacteria of skin also release acids and other metabolic wastes which the growth of pathogens on the surface of skin contain bacteria and fungicidal substance which kill the bacteria and fungi respectively. This shows that skin is a self-disinfecting organ.

Mucus coating of epithelium of respiratory, gastrointestinal and reproductive tracts also acts as physical barrier as helps in trapping the microbes going inside the body.

(b) Physiological Barriers:

The mucous membrane lining the nasal chamber, throat and trachea secretes polysaccharide and is bactericidal which contains lysozyme (a lysosomal mucolytic polysaccharide and is bacterial) which kills the bacteria by causing dissolution of their lipid membrane.

There are a number of chemical substances which kill the bacteria which tend to enter our body:

(i) Tears of lacrymal glands contain a bactericidal protein called lysozyme which prevents eye infection.

(ii) Nasal secretions also contain lysozyme capable of inactivating some viruses

(iii) Saliva of the salivary glands also contains lysozyme to kill the germs in the food.

(iv) HCI of the gastric juices of the gastric glands of stomach provides a strong acidic medium (1-2 pHs) to kill the bacteria in the food.

(v) Certain symbiotic gut micro-orgasms secrete antibiotics to kill pathogenic bacteria.

(vi) Bile (an alkaline juice with pH 8.0) of liver pre­vents the growth of bacteria on the chyme m duodenum.

(vii) Certain bacteria of vagina of human female stimulate the production of lactic acid from glycogen of the cells. Lactic acid kills the foreign bacteria.

(viii) Cerumen (ear wax) of ceremonious glands of auditory canal prevents the entry of insects. It also traps dust and bacteria.

(c) Chemical Compounds of Blood:

(1) Complement System:

It is a system about 20 types of different proteins present in the blood plasma. These are generally present in their precursor or inactive from but once activated, these assist or complete but once activated, these assist or complement body’s other defence system. Principal proteins of this system are C₁, to C₉, B and D which show a cascade pathway which, in turn is activated by an antigen-ant body reaction (Fig. 8.14) explained by W.B. Saunders 1977.

Figure 8.14 shows that complement products help in a number of functions like opsonization and Phagocytosis (C_3b); Dhaeocvtosis lysis of invading organisms (C_5b6789); agglutination (adhering of macrophages); neutralization of viruses, chemo taxis of neutrophils and macrophages (C_3a, C_4a, C_5a) activation of mast cells and basophils (C_3a, C_4a, and C_5a) to release histamine and other chemicals causing focal inflammation. In other words, complement system amplifies the inflammatory response. It also bores holes in the bacterial cell walls and plasma membrane so promoting K⁺ outflow and salts inflow to cause bursting of the bacteria.

(2) Cytokine Barriers:

Certain animal cells infected with viruses produce polypeptides called Cytokines. One type of cytokine, an interferon, diffuses to healthy neighbouring cells and stimulates them to produce biochemicals that block viral replication. When these cells become infected, the viruses are unable to take over the protein synthetic machinery to manufacture more of themselves. The spread of the infection halts.

Interferon is a protein produced by virus-infected cells. Interferon binds to receptors of noninfected cells, causing the cells to prepare for possible attack by producing substances that interfere with viral replication. Interferon is specific to the species; therefore, only human interferon can be used in humans. Although it once was a problem to collect enough interferon for clinical and research purposes, interferon is now made by recombinant DNA (deoxy ribonucleic acid) technology. These have been successfully used in treating influenza and hepatitis but have not been successful against cancer.

(3) Fever:

A fever protects non-specifically. A viral or bacterial infection stimulates white blood cells to proliferate, producing cells that secrete proteins that form a substance called endogenous pyrogen. Endogenous pyrogen resets the thermoregulatory centre in the brain’s hypothalamus to maintain a higher body temperature, which may be too high for the infecting microbes to tolerate.

Fever also counters microbial growth indirectly, because higher body temperature reduces the iron level in the blood. Since, bacteria and fungi need more iron so as the temperahire rises, their growth is inhibited in a fever-ridden body. Phagocytes also attack micro-organisms more vigorously when the temperature rises.

(B) Second Line of Defence:

It is also called macrophage system and is the non-specific internal defence. It is formed of phagocytic cells present within the tissues and the blood stream which ingest and destroy foreign particles. It is essential as certain pathogens enter the human body successfully by breaking through the first line of defence.

It includes two types of cellular barriers:

(i) Leucocytes (WBCs):

The polymprphonuclear leucocytes (PMNL), especially the neutrophils (about 62% of leucocytes so are most abundant) of the blood have the ability to come out of blood capillaries by amoeboid movements called diapedesis (Fig. 8 15) engulf and digest many of the bacteria that cause acute infections. So the neutrophils are commonly called the soldiers of the body. The process of engulfing bacteria by the cells of macrophage system, is called phagocytosis (Fig. 8.16).

(ii) Macrophages, macrophages are large amoeboid phagocytic cells which are found in most of connective tissues (called histiocytes), lymph nodes, spleen, reticulo-endothelium, liver and bone-marrow. Microglia of CNS also act as macrophages. These are able to engulf particulate material including micro-organisms, tissue debris, any colloidal substance, dead cells etc. So these are called scavengers of the body.

These may be ‘fixed’ or ‘wandering’. Fixed macrophages remain attached to the tissues for months or even years but when stimulated, these break away from their attachments and become mobile macrophages and show chemotaxis. Macrophages along with the lymphocytes form the lymphoreticular or reticuloendothelial system.

The macrophages of liver form part of the endothelial lining of the sinusoids and are caled ‘Kupffer cells’ (These can phagocytize a single bacterium in less than 0.01 second). In both spleen and bone marrow, macrophages have become entrapped by the reticular meshwork. In spleen, large number of macrophages are present along the trabeculae and are capable of phagocytosis of unwanted debris in the blood, especially old and abnormal red blood cells.

Similarly, large number of tissue macrophages are present in the alveolar walls. These can engulf tubercle bacelli, silica, dust particles, and even carbon particles. Macrophages may fuse to form a large multinucleate giant cell to capture a large sized foreign body. The macrophages differ from leucocytes: the former have much greater ability to engulf inorganic particulate materials and are also responsible for the phagocytosis but cannot synthesize antibodies.

Inflammatory Response:

Leucocytes and macrophages, which form second line of defence, always operate through inflammatory response. According to this response, when the micro­organisms like bacteria, viruses etc. enter the body tissues through some injury; these produce some toxic substances which kill tissue cells.

The damaged cells release histamine which causes the inflammation characterized by dilation of capillaries and small blood vessels surrounding the injury increasing the blood flow to the injured tissues; infected area becomes red, warm and swollen; and increase in permeability of the capillary wall. Plasma leaks into the tissue spaces so diluting the toxins secreted by the bacteria.

The phagocytes (neutrophils and macrophages) show chemotactic response and are attracted by chemicals released from inflamed area. Some of the many tissue products that cause these reactions are histamine, bradykinin, serotonin, prostglandins, several reaction products of the complement system and blood clotting system, and multiple hormonal substances called lymphokines of sensitized T-cells.

The phagocytes move towards infected or injured area, leak into the interstitial spaces and engulf the invading microbes. The dead microbes and blood corpuscles form the pus in the wound region which further increases the response of defensive system of body. This is called inflammatory response.

The intensity of the inflammatory response is usually proportional to the degree of tissue injury e.g. Staphylococci bacteria release extremely lethal toxins so initiating rapid development of inflammatory response and the infection is warded of rapidly but Streptococci bacteria do not cause intense local tissue destruction so inflammatory response is slow and are more harmful.

The inflammatory response may be only in the injured or infected region (localized), or it may spread all over the body (systemic). In systemic inflammatory response, the number of WBCs increases. The toxins released by the pathogens, or the pyrogens (Gr. pyre = fire) released by the leucocytes cause the fever.

This increase in temperature is called pyrexia. The moderate high temperature stimulates the phagocytic activity and inhibits the growth of micro-organisms as fever decreases the iron level in blood (iron promotes the growth of bacteria). Fever also increases the production of interferon. However, a very high rise in body temperature is dangeous so it should be quickly brought down by using antipyretics (Gr. anti = against; pyre = fever) like asprin and by applying cold packs.

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Essay # 2. Specific Defence or Immune System:

It is not present from the birth but is acquired during one’s own life span. It is developed by an organism in response to a disease caused by the infection of microbes or vaccine. It is due to the presence of an extremely powerful specific immune system which forms third line of defence.

The most peculiar characteristic of immune system is that it can differentiate between ‘self’ (body’s own cells) and ‘non-self’ (foreign microbes) It provides defence against invading agents such as lethal bacteria, viruses, toxins and even transplants In this protective lymphocytes of body produce antibodies which not only inactivate the antigens and relieve from an infectious disease but also provide immunity against further attack.