Through the use of absorbed idiotypic antisera prepared against single isolated monoclonal IgM anti-γ-globulins, partial cross-idiotypic specificity was demonstrated with other IgM anti-γ-globulins. Such antisera classified these proteins into at least three groups. The major group which included 60% of the anti-γ-globulins was particularly homogeneous. The anti-γ-globulin specific antigens were detected best in hemagglutination and hemagglutination inhibition systems. They were not found in monoclonal IgM proteins that lacked anti-γ-globulin activity although related antigens were detected at low concentrations in pooled immunoglobulin preparations as well as in heterogeneous anti-Rh antibodies.

Several lines of evidence were obtained indicating that the antibody combining site was involved in the specific determinants. Attempts were made to analyze the fine specificity of each anti-γ-globulin for the Fc fragment of different subclasses of human immunoglobulins as well as those of other species. Differences were observed but these were not readily related to the cross-specificity antigens.

The anti-γ-globulin specific antigens were very analogous to those previously described for monoclonal IgM cold agglutinins. Although each protein could be distinguished from all the others on the basis of individual idiotypic antigens, the antigens common to the specific groups of proteins with each of these activities were prominent and readily detected with multiple antisera. The results indicate basic similarities between proteins of a given activity even in unrelated individuals.

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