Component of the membrane attack complex (MAC), a multiprotein complex activated by the complement cascade, which inserts into a target cell membrane and forms a pore, leading to target cell membrane rupture and cell lysis (PubMed:17872444, PubMed:22832194, PubMed:26841837, PubMed:27052168, PubMed:30552328, PubMed:7440581). The MAC is initiated by proteolytic cleavage of C5 into complement C5b in response to the classical, alternative, lectin and GZMK complement pathways (PubMed:17872444, PubMed:30552328, PubMed:39914456, PubMed:39814882, PubMed:7440581). The complement pathways consist in a cascade of proteins that leads to phagocytosis and breakdown of pathogens and signaling that strengthens the adaptive immune system (PubMed:17872444, PubMed:30552328, PubMed:7440581). C8A, together with C8B and C8G, inserts into the target membrane, but does not form pores by itself (PubMed:17872444, PubMed:30552328). During MAC assembly, associates with C5b, C6 and C7 to form the C5b8 intermediate complex that inserts into the target membrane and traverses the bilayer increasing membrane rigidity (PubMed:30552328, PubMed:6833260). {ECO:0000269|PubMed:17872444, ECO:0000269|PubMed:22832194, ECO:0000269|PubMed:26841837, ECO:0000269|PubMed:27052168, ECO:0000269|PubMed:30552328, ECO:0000269|PubMed:39814882, ECO:0000269|PubMed:39914456, ECO:0000269|PubMed:6833260, ECO:0000269|PubMed:7440581}. This is the function of ENSG00000157131 (C8A, complement C8 alpha chain).