Heterogeneity of bovine IgG2--I. The A1 allotypic determinant is the major antigenic determinant recognized on bovine IgG2a by polyclonal rabbit anti-IgG2a.

Rabbit antisera specific for the IgG2a subclass of bovine immunoglobulins also contain antibodies which recognize the A1 allotype of this immunoglobulin subclass. In many sera anti-Al constitutes the major specificity of these rabbit antisera and all of the 10 rabbits immunized with Al(+) IgG2a produced some antibodies which recognized the allotype. The determinant is the same as recognized by a bovine alloantiserum to Al. The Al allotype is shared by the intact and the Fc portion of IgG2a, but rabbits immunized with the Fc fragment also recognize an Al-related determinant which is not exposed on intact Al(+) IgG2a. None of the four rabbits immunized with Al(-) IgG2a produced precipitating antibodies which recognized the gene product of the Al(-), i.e. A2 allele, suggesting that only Al is an immunodominant antigenic determinant for rabbits. Data reported here help to explain the antigenic heterogeneity seen among the IgG2 populations from different cattle when they are tested by immunoprecipitation using rabbit anti-IgG2 reagents.