Complement factor C9, 71kDa, is the last of the human complement components creating the membrane attack complex (MAC). Activation of the complement system leads to the formation of the MAC, which in turn causes lysis, and death of target cells. C9 plays an important role in inflammatory reactions since it is essential for lytic action of the MAC. Such lytic action could be the basis for complement autotoxicity. Like most of the complement components, C9 belongs to the acute-phase proteins which are generally of hepatic origin, and their plasma concentrations increase (or decrease) following tissue injury and inflammation. Also, because C9 is required for the MAC of complement to become functional, interfering with signaling pathways that stimulate its production could offer new therapeutic strategies for treating various neurodegenerative disorders.