Immunology

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Due to continuous exposure to various infective agents like viruses, bacteria and fungi and parasites present in the environment. The human body is provided with defence mechanism to protect it from various infections and this is called immunity. So, immunity can be defined as the body’s ability to resist the harmful effects of infective organisms or toxins. This study of the immunity system of the body is named immunology. It refers to all mechanisms used by the body for self defence against pathogenic microorganisms, their products and non-microbial antigens or toxins.

There are some organs responsible for immunity together constitutes the immune system of the body. It comprises of the lymphoid organs where are the lymphocytes undergo maturation and proliferation. There are two kinds of lymphoid organs, primary and secondary. Primary lymphoid organs are thymus and bone marrow where lymphocytes are processed and matured to form two distinct types called T-lymphocyte and B-lymphocyte respectively. After it become matured the T and B lymphocytes migrate via blood vascular and lymphatic systems to the secondary lymphoid organs and they remain stored their; and undergo rapid proliferation in response to antigenic stimulus. These organs include lymph nodes, spleen, tonsil and the gastrointestinal lymphatic tissue. The immunology studies all about the systems of immunity in human beings. When an infective agent that called antigen, invades the body, the T and B lymphocytes Road in secondary lymphoid organs become activated and they propagate rapidly fight out the invader.

What is antigen and antibody?

In this study of immunology the antigen and antibody are two important words. Antigens are foreign substances which on entering into the body evoke immune response and are responsible for specific reaction with the antibodies. Chemically, antigens are usually proteins in nature or may be combination of proteins with other substances like polysaccharides, lipids, nucleic acid or some simply molecules like Haptens. Haptens are the substances which are unable to induce antibody formation but are able to react specifically with antibodies. Haptens are capable to induce antibody production on combining with larger molecules usually proteins. So haptens are played an important part in immunology because it is that protein of an antigen which can determine the immunologic specificity but is not self-sufficient to produce immune response. Due to this haptens are considered as complete antigens.

On the other hand and antibody is a protein produced in the body in response to entry of an antigen and is capable of reacting specifically with the same antigen to inactive it. So it can be said that antigens are antibody generators and antibodies are antigen destroyers. The antibodies are produced by the action of plasma cells formed from B-lymphocytes in response to stimulation by specific agents. As they are derived from bone marrow it is named B-lymphocyte.

The immunology divides antibodies in five types according to their functionality, those are: –
1) Antitoxins: it is the inactive toxin.
2) Aggulutinins: It causes agglutination (clumping) of cells containing the antigen.
3) Precipitins: These are causes precipitation of flocculation of soluble antigens.
4) Cytolysins: it causes lysis or break down cough antigen containing cells.
5) Opsonins: this type of antibodies causes opsonisation (coating) of antigen containing cell to facilitate the phagocytosis.

The antibodies are also known as immunoglobulin or I_gs because the belonging of them to the ɣ globulin fraction of plasma proteins having immune function. The basic structure of each I_g molecule consists of four polypeptide chains. Two numbers of long chains called heavy chains and two numbers of short chains called light chains. These four chains are held together by S-S (disulphide) bonds forming a Y-shaped molecule. The I_g molecule has a constant part and the variable and the variable part is located at the tip of the two arms of the Y. The two arms of the Y contains the antigen binding site binds with the specific antigen in a lock and key manner which forms an antigen antibody complex, and thus the antigen is destroyed.

As per immunology system, on the basis of their chemical, physical and physiological nature, immunoglobulins are classified into five groups, those are as follow: –

I_gA: in the body secretion such as saliva, tears, milk and all mucous membrane secretions. It provides primary defence mechanism at the mucous membranes against any local infections. Passive immunity may obtained by the baby from the mother’s milk.
I_gD: mainly it is found on the plasma membrane of lymphocytes and probably act as antigen receptors.
I_gE: allergic reactions are mediates by it.
I_gG: it is the major antibody which is present this is and plays the most important role in inactivation of toxin and the invading microorganisms like bacteria and virus.
I_gM: on the earliest exposure is to antigen I_gM is produced. Presence of it may indicate a recent infection and it helps in clumping of antigen containing cells.
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