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 (PubMed:26841934, PubMed:9212048, PubMed:9634479). C9 is the pore-forming subunit of the MAC (PubMed:26841934, PubMed:30111885, PubMed:4055801).
