Platelet associated complement components (PAC3c and PAC3d) in patients with autoimmune thrombocytopenia

Platelet associated C3c and C3d (PAC3c and PAC3d) were quantitated by enzyme linked assay in 105 patients with idiopathic autoimmune thrombocytopenia (AITP) in whom elevated platelet associated immunoglobulins (IgG and IgM) had previously been documented. Increased levels of complement components were demonstrated in 46 of 105 patients (43.8%). In 11 of these patients, PAC3d alone was abnormal implying that C3b had been inactivated after cleavage by C3 inactivator in vivo. Complement binding was seen in two of 16 patients (12.5%) with raised PAIgG alone, four of 19 patients (21.0%) with raised PAIgM alone and in 40 of 70 patients (57.1%) in whom PAIgG and PAIgM were raised together. This difference was highly significant (P = 0.01). The clinical implication of these findings are discussed.