Platelet Antibodies in Thrombocytopenic Patients

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Abstract
Platelet antibodies bound to the surface of platelets or free in the serum were sought in patients who had platelet counts for a variety of reasons. They were detected by finding excess Ig[immunoglobulin]G on the surface of washed platelets directly or after incubation of the serum with normal platelets. The technique used was a modification of one recently described in which the greater the amount of anti-IgG consumed by the reaction with platelets the less the subsequent lysis of sheep red cells coated with IgG. This test could be calibrated by adding known quantities of IgG to the antisera and thus the amount of bound IgG could be measured. Platelets from normal donors and those with thrombocytopenia due to non-immunological causes such as aplastic anemia or acute leukemia had 15-70 ng IgG/107 platelets (mean 53 ng). Of 38 thrombocytopenic patients in whom immune destruction of platelets was suspected, 37 had excess IgG on their platelets ranging from 70-720 ng/107 (mean 297 ng, P < 0.001) and there was a significant inverse correlation between this amount and the platelet count (r [correlation coefficient] = 0.85, P < 0.001). Antibody in the serum was found in 14 of 22 patints with idiopathic thrombocytopenic purpura (ITP), 3 of 4 patients with underlying lymphoma and in all 5 cases of systemic lupus erythematosus (SLE). Four non-thrombocytopenic patients with autoimmune hemolytic anemia (AIHA) due to IgG on the red cells had no increase in platelet-bound IgG. Platelet antibody as excess IgG can be readily detected on the surface of platelets in patients with immune thrombocytopenia. The clinical implications of these findings are discussed.

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