Immunoglobulin Antigen Determinants
There are three major types of immunoglobulin antigen determinants: isotypes, allotypes, and idiotypes.
The isotype of an immunoglobulin refers to the particular con-stant region of the light- or heavy-chain of the immunoglob-ulin. Immunoglobulins are classified on the basis of various heavy chain isotypes. Heavy chains are distinguished by the presence of heavy chain markers, such as , , , , and in the immunoglobulins IgM, IgG, IgA, IgD, and IgE, respectively. The light chains are also distinguished by isotype markers, such as and . Isotypes are present in all members of a species.
The allotype refers to allelic differences in both the variable and constant regions of immunoglobulin. The allotype markers are present on the constant regions of light and heavy chains. They are Am on heavy chains, Gm on heavy chains, and Km on light chains. Allotype markers are absent on , , and heavy chains and on light chains. More than 25 Gm types, 3 Km allotypes, and 2 Am on IgA have been described. Allotypes are present in some but not all members of a species and are inherited in a simple Mendelian fashion.
The idiotype refers to a specificity that is associated with the variable region. Idiotype markers are found on the hypervari-able region of the immunoglobulin. Idiotypes are specific for each antibody molecule. Anti-idiotypic antibodies produced against Fab fragments prevent antigen–antibody interaction.