Dual enzyme activation of C5. C5b, the product of C5 activation, has been regarded as the exclusive initiator of MAC complex assembly (left: C5 activation by complement only). C5 convertases and thrombin are often simultaneously formed in response to many pathologies in vivo to maintain homeostasis. We propose that activation of C5 proceeds via 2 pathways using either C5_T or C5b as reaction intermediates and C5a and C5b_T as final reaction products. C5b,6 that is formed is probably rapidly converted to C5b_T,6.