An antigenic determinant is the specific chemical determinant group or molecular configuration against which the immune response is directed. Because they are proteins, immunoglobulins themselves can function as effective antigens when used to immunize mammals of a different species. When the resulting antiimmunoglobulins or antiglobulins are analyzed, three principal categories of antigenic determinants can be recognized— isotype, allotype, and idiotype (Fig. 1; Table 1).

Fig1. Variants of antibodies—antigenic determinants.  (Adapted from Turgeon ML: Fundamentals of immunohematology, ed 2, Balti more, 1995, Williams & Wilkins.)

Table1. Immunoglobulin Variants

Isotype Determinants

The isotypic class of antigenic determinants is the dominant type found on the immunoglobulins of all animals of a species.

The heavy-chain, constant region structures associated with the different classes and subclasses are termed isotypic variants. Genes for isotypic variants are present in all healthy members of a species. Determinants in this category include those specific for each Ig class, such as gamma (γ) for IgG, mu (µ) for IgM, and alpha (α) for IgA, as well as the subclass-specific determinants κ and λ.

Allotype Determinants

The second principal group of determinants is found on the immunoglobulins of some, but not all, animals of a species. Antibodies to these allotypes (alloantibodies) may be pro duced by injecting the immunoglobulins of one animal into another member of the same species. The allotypic determinants are genetically determined variations representing the presence of allelic genes at a single locus within a species. Typical allotypes in humans are the Gm specificities on IgG (Gm is a marker on IgG). In humans, five sets of allotypic markers have been found—Gm, Km, Mm, Am, and Hv.

Idiotype Determinants

A result of the unique structures on light and heavy chains, individual determinants characteristic of each antibody are called idiotypes. The idiotypic determinants are located in the variable part of the antibody associated with the hypervariable regions that form the antigen-combining site.