Alloantibodies against the H-2b haplotype were produced in C3HeB female and male parental mice in response to transplantation of F1 fetal tissue (bones). In testing IgG1 and IgG2 antibodies by means of the FACS II we found striking sex-associated differences in the isotypes produced: female mice produced much more IgG1 than male mice, although they produced comparable amounts of IgG2. Using protein-A-sepharose-purified fractions of IgG1 and IgG2a, raised in the parental females we found the specificity of this extra IgG1 to be directed against the H-2Kb private determinants, in contrast to theIgG2a which was found to be specific for H-2Kb, H-2Db and I-Ab subregion alloantigens. The relevance of the association between H-2K alloantigens and IgG1, in context of the fetal-maternal relationship is discussed.
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