complement protein molecule [L, complementum + proteios, first rank] , any of the protein molecules that are chief humoral mediators of antigen-antibody reactions in the immune system. Complement proteins stimulate phagocytosis and inflammation. Nine are involved in the “classical pathway” cascade that results in the lysis of antibody-coated bacteria. They are designated C1 to C9. Overall, more than 30 complement proteins have been identified so far.
