A receptor for IgA was purified from human polymorphonuclear neutrophils (PMN) by affinity chromatography on human serum IgA-Sepharose. The receptor appeared on SDS/polyacrylamide gels as a diffuse band with an apparent molecular mass of 50-70 kDa, whether reduced or non-reduced. During purification, the protein showed remarkable stability to proteolytic digestion by endogenous PMN proteinases. Purified radioiodinated receptor re-bound to IgA-Sepharose, but not to IgG-Sepharose or BSA-Sepharose. The binding of the receptor to IgA-Sepharose was inhibited in a dose-dependent manner by human serum IgA1 or IgA2 or secretory IgA1 or IgA2, but not by IgG or IgM. Binding of receptor to IgA-Sepharose was also inhibited by the Fc fragment of IgA, but not by the Fab fragment. An IgA fragment produced by digestion with pepsin which lacks the CH3 domain also inhibited binding, but to a more limited extent than did the whole IgA molecule.

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