Although all members of a species inherit the same set of isotype genes, multiple alleles exist for some of the genes (see Figure 4-17b). These alleles encode subtle amino acid differences, called allotypic determinants, that occur in some, but not all, members of a species. The sum of the individual allo-typic determinants displayed by an antibody determines its allotype. In humans, allotypes have been characterized for all four IgG subclasses, for one IgA subclass, and for the k light chain. The 7-chain allotypes are referred to as Gm markers. At least 25 different Gm allotypes have been identified; they are designated by the class and subclass followed by the allele number, for example, G1m(1), G2m(23), G3m(11), G4m(4a). Of the two IgA subclasses, only the IgA2 subclass has allotypes, as A2m(1) and A2m(2). The k light chain has three allotypes, designated Km(1), Km(2), and Km(3). Each of these allotypic determinants represents differences in one to four amino acids that are encoded by different alleles.
Antibody to allotypic determinants can be produced by injecting antibodies from one member of a species into another member of the same species who carries different allo-typic determinants. Antibody to allotypic determinants sometimes is produced by a mother during pregnancy in response to paternal allotypic determinants on the fetal im-munoglobulins. Antibodies to allotypic determinants can also arise from a blood transfusion.
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