Pathway which activates the proteins of the complement system, a group of blood proteins of the globulin class involved in the lysis of foreign cells after they have been coated with antibody, and which also promote the removal of antibody-coated foreign particles by phagocytic cells. The pathway proceeds by a cascade reaction of successive binding and proteolytic cleavage of complement components. This pathway can be activated by either IgG or IgM binding to an antigen.