PLATELET-BOUND COMPLEMENT (C3) IN IMMUNE THROMBOCYTOPENIA

Abstract

The fixation of complement [C] to the circulating platelet in immune thrombocytopenia was detected by measurement of C3 on the surface of platelets from patients with idiopathic thrombocytopenic purpura (ITP) and systemic lupus erythematosus (SLE) using the anti-C3 consumption assay. The surface Ig[immunoglobulin]G was determined simultaneously using the previously described anti-IgG consumption assay. Washed platelets from normal controls had 3.5 fg (10-15 g) of C3, or about 11,000 molecules per platelet, an amount comparable to IgG (4.1 fg, or 15,000 molecules per platelet). For most patients with ITP, C3 and IgG were increased on the platelet surface, although for 5 of 16 patients only IgG was increased. Two patients with SLE and thrombocytopenia had an increase in C3 and IgG. Six patients with SLE who were not thrombocytopenic had normal amounts of membrane-bound C3 and IgG. In 5 patients, 3 with ITP and 2 with collagen vascular disease, both surface immunoproteins decreased with successful treatment of the thrombocytopenia.