7.3.6 Antibodies

. . Antibodies Antibodies are immunoglobulin (Ig) protein molecules synthesized on exposure to antigen that can combine specifically with the antigen. Whenever pathogens enter our body, the B-lymphocytes produce an army of proteins called antibodies to fight with them. Thus, they are secreted in response to an antigen (Ag) by the effect of B cells called plasma cells. The antibodies are classified into five major categories, based on their physiological and biochemical properties. They are IgG (gamma), IgM (mu), IgA (alpha), IgD (delta) and IgE (epsilon). In the 1950s, experiments by Porter and Edelman revealed the basic structure of the immunoglobulin. An antibody molecule is Y shaped structure that comprises of four polypeptide chains, two identical light chains (L) of molecular weight , Da (approximately amino acids) and two identical heavy chains (H) of molecular weight , Da (approximately amino acids). The polypeptide chains are linked together by di-sulphide (S-S) bonds. One light chain is attached to each heavy chain and two heavy chains are attached to each XII Std Biology-Zoology Chapter- XII Std Biology-Zoology Chapter- Human Health and Diseases other to form a Y shaped (Fig. . ) structure. Hence, an antibody is represented by H L . The heavy chains have a flexible hinge region at their approximate middles. Antigen binding site Variable region Disulphide bond Constant region Light chain Heavy chain