Complement component 9 (C9) is a MACPF protein involved in the complement system, which is part of the innate immune system.[5] Once activated, about 12-18 molecules of C9 polymerize to form pores in target cell membranes, causing lysis and cell death.[6] C9 is one member of the complement membrane attack complex (MAC), which also includes complement components C5b, C6, C7 and C8.[7][8][9] The formation of the MAC occurs through three distinct pathways: the classical, alternative, and lectin pathways.[7] Pore formation by C9 is an important way that bacterial cells are killed during an infection, and the target cell is often covered in multiple MACs. The clinical impact of a deficiency in C9 is an infection with the gram-negative bacterium Neisseria meningitidis.[10]
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