Heavy-Chain Subclass of Bound Antiplatelet IgG in Autoimmune Thrombocytopenic Purpura

The gamma heavy-chain subclass of bound antiplatelet antibody was examined in six patients with autoimmune thrombocytopenic purpura (ATP) by a solid-phase radioimmunoassay. Monospecific antisera for ‘yGl , yG2, ‘yG3. and ‘yG4 subclasses were employed in a “sandwich” technique, utilizing the binding of 125l-staphylococcal protein A. We have previously reported that serum antiplatelet antibody was restricted to the ‘yG3 subclass in ATP. In contrast, all 4 IgG subclasses were found bound to platelets of ATP patients in the same distribution as that present in normal serum. It is suggested that the differences noted between serum antiplatelet IgG and platelet-bound IgG may represent different mechanisms of platelet injury.