Platelet-associated complement C3 in thrombocytopenic states

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Abstract
Platelet-associated complement components C3, C3d and C4 (PAC3, PAC3d, PAC4) were quantitated by a modification of the platelet radioactive anti-IgG test using highly purified C3 antibodies in 74 patients with immune thrombocytopenia (ITP), 26 patients with presumed nonimmune thrombocytopenia (NTP) and 114 normal individuals. Elevated PAC3 levels were found in 26 out of 74 patients with ITP (35%) and in 10 out of 26 patients with NTP. Although the percentage of elevated PAC3 values was higher in thrombocytopenic patients than in nonthrombocytopenia patients, no statistically significant correlation existed between platelet counts and PAC3 levels, neither for ITP nor for NTP. Such a relationship was demonstrable between PAC3 and platelet-associated IgG, both for ITP (P < 0.05) as well as NTP patients (P < 0.001). Elevated PAC3 values apparently are not restricted to immune thrombocytopenias. Quantitative differences of PAC3 between ITP and NTP patients suggest that part of the PAC3 is immunologically mediated and has a role in the pathogenesis of autoimmune thrombocytopenias.