Human triclonal anti-IgG gammopathy. II. Determination of the antigenic specificity patterns of the IgG, IgA and IgM autoantibodies for the subclasses of IgG.

The specificity and reactivity patterns of monoclonal IgG, IgA and IgM anti-IgG autoantibodies isolated from the serum of one patient (Gil) have been determined for IgGs of the four gamma chain subclasses. The haemagglutination produced by the interaction of the Gil anti-IgGs and anti-Rh IgG coated erythrocytes was inhibited by a panel of intact IgGs, their polypeptide chains, and enzymatic fragments which included purified heavy chain constant region domains. Intact IgG1, IgG2, and IgG4 produced the same patterns of reactivity with the Gil anti-IgGs. When partially reduced and alkylated IgG1 heavy chains and its tryptic digests were tested, these were much more reactive than Fc fragments isolated from IgG of the four subclasses which were weaker inhibitors, and gamma chain constant region domains which were totally non-reactive. In all instances and by use of two anti-Rh antisera, the specificity patterns obtained for the Gil anti-IgGs were identical. The data combined with previous knowledge of the identity of the Gil light chains suggests that the antibody combining sites of these molecules are very similar if not identical.