Distribution of Antigenic Determinants on Human IgA1

. The distribution of antigenic determinants on human IgA was studied with fragments and mutants of IgA1. F(abc)2 and Fabc, lacking the CH3 domain, F(ab′)2 and Fab′, lacking the CH2 and CH3 domain, Fab that further lacks most of the hinge region, and Fc fragments were included in our investigations. Antibodies specific for the CH3 domain of IgA1 were found in antisera raised against an α1-HCD protein and in anti-Fcα antisera. The antisera detected different antigenic determinants on CH3 as was shown by inhibition with sera from nonhuman primates. An anti-Fc5μ antiserum detected a determinant on Fc common to IgA and IgM. The serum of an IgA-deficient individual reacted with a determinant on CH2 of four-chain molecules only. A subclass-specific anti-Fabc antiserum detected a determinant which needed interaction of CH2 and CH1 or the hinge region. Anti-Fab antisera reacted with class or subclass specific determinants on CH1. The isoal-lotype nA2m(2) is probably located on CH1. Its expression requires two α-chains stabilized in a conformation attributed to by CH2.