A variety of complement-activating substances, including inulin, immunoglobulin aggregates, bacterial endotoxins, and staphylococcal protein A, were found to initiate blood coagulation through a complement-mediated pathway. These substances markedly accelerated blood coagulation in normal rabbit blood. That this clot-promoting activity requires an intact complement system was demonstrated by an almost total lack of effect on blood from rabbits with an inherited deficiency of the sixth component of complement (C6). Small amounts of isolated C6 conferred to C6-deficient blood the ability to respond with accelerated coagulation upon activation of the complement system. In addition, it was determined that activation of complement through the previously described C3 activator system resulted in the initiation of blood coagulation. The participation of C1, C2, and C4 was not necessary.

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