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). 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. 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.