Constituent of the membrane attack complex (MAC) that plays a key role in the innate and adaptive immune response by forming pores in the plasma membrane of target cells. C9 is the pore-forming subunit of the MAC. Component of the membrane attack complex (MAC). MAC assembly is initiated by proteolytic cleavage of C5 into C5a and C5b. C5b binds sequentially C6, C7, C8 and multiple copies of the pore-forming subunit C9. About 20 C9 chains oligomerize to give rise to a huge beta-barrel that forms a 100 Angstrom diameter pore in target membranes. Secreted as soluble monomer. Oligomerizes at target membranes, forming a pre-pore. A conformation change then leads to the formation of a 100 Angstrom diameter pore. Thrombin cleaves factor C9 to produce C9a and C9b. The structure of the human polymeric form indicates the existence of an additional disulfide bond compared to the mouse monomeric form. Belongs to the complement C6/C7/C8/C9 family.