sentences:  As noted, C4 (mixture of C4A and C4B) participates in all three of the complement pathways (classical, alternative, and lectin); the alternative pathway is "triggered spontaneously," while the classical and lectin pathways are elicited in response to the recognition of particular microbes.All three pathways converge at a step in which complement protein C3 is cleaved into proteins C3a and C3b, which results in a "lytic pathway" and formation of a macromolecular assembly of multiple proteins, termed the membrane-attack complex (MAC), which serves as a pore in the membrane of the targeted pathogen, leading to invading cell disruption and eventual lysis.In the "classical pathway," the complement component—hereafter abbreviated by the "C" preceding the protein number— termed "C1s," a serine protease, is activated by upstream steps of the pathway, resulting in its cleavage of the native, parent ~200 kilodalton (kDa) C4 protein—composed of three chains.