The antigenic heterogeneity of bovine IgG2 observed by single radial diffusion, when different cattle sera are tested with a panel of IgG2-specific reagents, was examined by a combination of biochemical and serological assays. Using an autologous anti-A1 allotypic reagent, the major antigenic heterogeneity detected by swine, rabbit and goat anti-IgG2 reagents was due to their propensity to recognize the AI allotope. All heterologous reagents and most monoclonals so far test are biased in their specificity toward this determinant. A second type of serological heterogeneity, recognized by only certain heterologous reagents, was their specificity for what we have called the IgG2b isotype of IgG2. This isotype is found in all cattle, has a restricted ion-exchange elution behaviour and does not bear a-locus allotypic determinants; molecules bearing the latter are now designated IgG2a. IgG2a elutes from anion-exchange columns in subpopulations over a wide range of ionic strength including fractions which contain IgG2b and IgG1 as well. Using IgG2a from an AI homozygous steer, these subpopulations were shown to result from idiotypic variation which appears to primarily reside in their VH-regions.
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