3.3. Antibodies (= Immunoglobulins)
3.3.1. Antibody Is Immunoglobulin
Immunoglobulin. Antibodies (Ab) are high-molecular weight (140,000 - 1000,000) soluble proteins (immunoglobulins) produced by organisms in response to foreign substances, antigens (Ag), with whom they form immunochemical complexes:
Production of Antibody. The immune system consists of B cells and T cells in the blood serum. The B cells produces antibodies and when antibody graps onto antigen (the foreign invader), T cells destroy the antigen. Each antibody-producing B cell is programmed to make just one antibody, which is placed on its surface as an antigen receptor. Each B cell has a different antigen binding specificity (1-n in Fig. 3.10). Antigen binds to only those B cells with the appropriate surface receptor. These cells are stimulated to prolifereate and mature into antibody-producing cells and the longer-lived memory cells, all with the same antigen binding specificity (2 in Fig. 3.10).
Structure of Antibody. The basic structure of all immunoglobulin molecules is a unit consisting of two identical light polypeptide chains and two identical heavy polypeptide chains linked together by disulphide bonds (Fig. 3.11). The class and subclass of an immunoglobulin molecule is determined by its heavy chain type. Thus the four human IgG subclasses (IgG1, IgG2, IgG3 and IgG4) have heavy chains called g1, g2, g3, and g4 which differ only slightly although all are recognizably y heavy chains. The differences between the various subclasses within an immunoglobulin class are less than the differences between the different classes; thus IgG1 is more closely related to IgG2, 3, or 4 than to IgA, IgM, IgD or IgE.
Subclasses of Antibodies. The four subclasses of human IgG occur in the approximate proportions of 66, 23, 7 and 4 per cent respectively. There are also known to be subclasses of human IgA (IgA1 and IgA2) but none have been unambiguously described for the other classes. Immunoglobulin subclasses appear to have arisen after speciation and the human subclasses cannot be compared with, for example, the four known subclasses of IgG which have been identified in the mouse.