Hemolytically active human C1q incubated with EA before the addition of complement inhibited the immune hemolysis. On the contrary, heat-inactivated preparation (30 min 56°C) was ineffective. Preincubation of EA with bovine collagen also resulted in a decreased hemolysis. When aggregation was measured by a turbidimetric method in citrated human platelet-rich plasma, it was found that hemolytically active human C1q (C1) alone does not induce platelet aggregation. However, in its presence the

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... elets failed to aggregate or exhibited a significantly reduced aggregation response to bovine collagen. The inhibition by C1q depended on the preincubation time with platelets. Heat treatment (30 min 56°C) destroyed the inhibitory action of C1q (C1). The effect of C1q proved to be highly specific because different C1q preparations at their inhibitory doses in collagen-induced platelet aggregation did not influence the response to other aggregating agents (bovine thrombin, ADP, horse anti-human thymocyte globulin, goat anti-baboon platelet antiserum). The results prove that collagen and C1q are capable of binding to the same site(s); namely, to those of EA and human platelets; furthermore, they suggest the presence of a receptor for C1q (C1) on human platelets.