Immune system, ‘The Marshal of Defenders’

It is well known fact that microorganisms, too small to be seen through our naked eyes are widely dispersed in our environment. Their proximity of existence lies from air to water, root hair to fruit body and from deep ocean floors to top of mountains. Besides having them so widely dispersed (very few of them are infectious) in our environment, why don’t we all get infected or suffer from diseases too often? The key role player behind this lies within our body: the Immune System.

The immune system comprises of hair-like cilia of lower respiratory tract to specialized B-cells and T-cells. The body’s defense mechanism i.e. the immune system may be of either Innate type or Adaptive type. The innate immunity also called as Natural Immunity is the resistance possessed by an individual by birth. They are non-specific in nature, which means, instead of targeting a specific type of invader, they actually act on any substance foreign to our body. Skin and mucus membrane are two important parts of innate immunity. The pH of skin maintained between 3 and 5 is inhibitory to most microorganisms and the sebum secreted by sebaceous gland of skin contains Lactic acid and Fatty acids which too prevents the growth of microorganisms. Mucus on the other hand does the same job by entrapping invading microorganisms which on the lower respiratory tract, later, is propelled out by synchronous movement of cilia. Secretions like saliva and tear contain Lysozyme which attacks on peptidoglycan layer of bacterial cell-wall. It is seen that only few microorganisms can tolerate the highly acidic environment of stomach. Beyond these barriers of Innate immunity lie a special type of cells called ‘Phagocytes’, demonstrated by Metchnikoff. These cells simply ‘eat-up’ invading microorganisms and slaughter them inside. Complement; a group of serum proteins circulating on blood plasma can damage the membranes of pathogenic microorganisms and stand up as one of the important part of innate immunity.

Antibodies attaching on
bacterial surface receptor

The Adaptive immunity (also referred to as Acquired Immunity) is the resistance acquired by an individual during the life. Components of adaptive immunity are capable of recognizing and selectively eliminating specific foreign molecules. The adaptive immunity generally comes into play after an organism has breached the barriers of innate immunity. This type of immunity is induced after the exposure to specific antigens (any foreign substance able to evoke immune response) and by administration of preformed antibodies (globular proteins that specifically bind to antigens). The former is referred to as active adaptive immunity and the latter as passive adaptive immunity. The

active immunity is induced by natural clinical and sub-clinical infections and by vaccination. Vaccines are the preparations of live, attenuated or killed microorganisms, or their antigens or active materials derived from them. The first exposure of pathogens to body by infections or through vaccine elicits a primary immune response and the pathogen is memorized then after by immunological memory. If again the pathogen is introduced into the body, the immunological memory helps to confer a strong secondary immune response against the pathogen. On the other hand passive immunity is induced in an individual by administration of preformed antibodies raised against an infecting agent (microorganisms).

The immune system has its own troop of specialized cells that actively combat against invaders. Thymus-derived T-cells, bone marrow-derived B-cells, Natural killer cells, Macropahges, Dendritic cells comprise the cells of immunity. T-cells maturing on thymus activate the B-cells which mature within bone-marrow. The B-cells after activation form the plasma cells and the memory cells. The plasma cells ultimately form another important component of immunity, the globular proteins called Antibodies. The antibodies specifically react with antigens and facilitate others in killing them. The macrophages being a phagocytic cell ‘eats-up’ the pathogens. The dendritic cells perform in the similar manner like that of macrophages.

Infection and disease causation are the result of declined immunity or increased pathogenesis of microorganisms that makes them able to breach the barriers of immunity. Mechanical barriers like skin and cilia to specialized cells of immunity like T-cells and B-Cells are actively defending our body against the invading microorganisms. They are protecting us from potential infections and diseases even we are continually exposed to microorganisms present everywhere in the environment. Thus, the Immune system is a perfect ‘Marshal of Defenders’.

(Content by Saroj Khadka, Administrator of IATSOM blog. He is currently pursuing Master's Degree in Medical Microbiology at University Campus of Tribhuvan University. Find him on twitter @microbiosaroj. Email: khadkasaroj09@gmail.com)