eChapters

The study of the immune system, in its various aspects, is called immunology. Immunology is the branch of bioscience which deals with the defense mechanisms including all physical, chemical and biological properties of the organism that help it to combat its susceptibility to foreign agents and microorganisms. Immunology includes the development and functions of various components of immune system by which the body responds, destroys or neutralizes foreign agents or pathogens. It includes the study at cellular as well as molecular level. Immunology is an interdisciplinary subject which incorporates and requires the knowledge of many branches of biology like microbiology, biochemistry, molecular biology and clinical medicine.

The major components of the immune system are the lymphocytes which constitute lymphoid system (Figure 2.1).The lymphoid organs are classified into two categories: primary and secondary lymphoid organs.

Innate immunity or innate immune response is a set of non-specific or less specific mechanisms which provides the first and rapid line of defense. All such mechanisms exist prior to the exposure to a pathogen i.e. before the onset of the infection. It is also called inborn or innate immunity. The different components of the innate immunity like physical barriers, chemical barriers, phagocytosis, inflammation etc. act to prevent the entry or elimination of the microorganisms in a non- specific manner.

Innate immunity provides the first line of defense against invading pathogens. But if the pathogens are able to escape from the first line of defense mechanisms, the body uses, another set of immune mechanisms which provide defense in a more specialized manner. These are called second line of defense mechanisms. The second line of defense mechanisms are highly specific in nature. These are called adaptive immune response and provide adaptive immunity. It is also called acquired immunity since these immune responses are not present prior to the exposure of the microbes. When the body is exposed to foreign substances or microorganisms, body develops adaptive immune responses. Adaptive immunity is a set of specialized cells and processes that function to eliminate pathogens or prevent their growth.

When the body is exposed to a microbe or a foreign substance, the body makes some response which is called immune response. Immune responses are initiated by the interaction between a ligand, present on the surface of the microbe or secreted by the microbe or a foreign substance, and a receptor protein on the cell’s surface or a soluble receptor. There are several factors such as shape, charge and presence of other receptors which influence this interaction between the ligand and receptor. This ligand which is recognized by the immune system is called antigen. An antigen can be defined as an organism, a molecule, or part of a molecule which is recognized by the immune system. Often, the term immunogen is also used for antigen. Though, antigen and immunogen are used as synonyms but specifically, they are different from each other.

Antibodies, also called immunoglobulins, are the proteins produced by the immune system against antigens. These are the protective proteins which interact with antigens and neutralize their effects. Antibodies are produced by B lymphocytes. An immunoglobulin has a Y-shaped structure consisting of four polypeptide chains.

Humoral immune response is the immune response which is mediated by antibodies.. It is called humoral immune response because antibodies are transferred with the ‘humour’ that is fluid or blood. Antibodies are the proteins produced by B lymphocytes.

Cell mediated immune response (CMI) is the major defense mechanism against intracellular microbes and tumor cells. It is mediated mainly by T lymphocytes. T cells originate in bone marrow but the maturation of T cells occurs in thymus.

The interactions between antigens and antibodies are specific in nature in which an antibody recognizes an antigen through complementarities of shapes on paratope and epitope. An antigen reacts only with antibodies produced by itself or with closely related antigens. The reaction is similar to a lock and key. As a standard lock can be opened by its own key only similarly, an antibody can react only with its own antigen. Antibodies recognize molecular shapes (epitopes) on antigens.