Cytolysis of a Human Cell with
Foreign Epitopes on its Membrane
Typically to activate the classical complement pathway, IgG or IgM is made in response to an antigen. The Fab portion of IgG (2 molecules) or IgM (1 molecule) reacts with epitopes of that antigen. This alters the shape of the Fc portion of the antibody. A protein called C1q is then able to bind to the Fc portion of antigen-bound IgG or IgM after which C1r and C1s attach. Collectively, these three proteins form C1, the first enzyme of the pathway. At the end of the complement pathway, C5b6789n, or the membrane attack complex (MAC), puts pores into the lipid bilayer membrane of the infected cell causing its lysis.