The complement system is a part of the larger immune system and three biochemical pathways are present: the classical complement pathway, the alternative pathway, and the mannose-binding lectin pathway. Human complement factor C6 is one of five components (C5b, C6, C7, C8, and C9) that interact to form the cytolytic membrane attack complex (MAC) which is the cytolytic end product of the complement cascade. MAC is typically formed on the surface of intruding pathogenic bacterial as a result of the activation of the complement system, and it is one of the ultimate weapons of the immune system. The sixth component of complement, C6, is a 913 amino acid single polypeptide chain serum glycoprotein. Homology study suggests that C6 could contain two domains, an amino-terminal region that is related to complement C8 and C9, and a carboxyl-terminal region that has partial homology to the complement regulatory proteins factor H and factor I. Genetic deficiencies of terminal complement components lead to markedly increased susceptibility to only one particular Gramnegative genus, the Neisseria. The susceptibility is attributable to the major role played by complement-mediated killing in host defense against the pathogen.