C9 is synthesised in the liver and monocytes, and is a plasma protein consisting of a single polypeptide chain of molecular weight 71kDa. Normal plasma concentration is 60mg/L. C9 forms part of the membrane attack complex (MAC) the cytolytic terminal complex of the complement pathways. C9 binds to the membrane associated C5b-8, binding of C9 to C5b-8 leads to the circular polymerisation of 12-18 C9 molecules. This is the basis of the hydrophilic, protein-walled, trans-membrane channel formed by the MAC, which leads to cell lysis and destruction.
