Human serum antibodies which react only with antigenic determinants revealed by digestion of IgA with pepsin were found in 29.2 per cent of the sera from 369 hospitalized patients studied. These antibodies were shown to be specific for sites on the F(ab')2a fragment of IgA and were dependent upon intact disulfide bonds for antigenicity. Considerable variability in reactivity with different pepsin-digested IgA proteins was found in individual sera but no relationship to either IgA subgroups or genetically determined IgA antigens was discerned. The presence of these anti-IgA antibodies was not found to be related to specific disease processes or to alterations in serum immunoglobulin concentrations.

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