Molecular model of immunoglobulin M (IgM, light blue) that is bound to an antigen (magenta) activating the C1 complex (violet) of the complement system. The complement system is a part of the innate immune system that complements the action of antibodies and phagocytic cells. When stimulated by one of several triggers, the end result of complement activation is the stimulation of phagocytes to clear foreign and damaged material, inflammation to attract additional phagocytes and activation of the cell-killing membrane attack complex.