Complement Antibodies bound to an invading microorganism activate the complement system via the classical pathway. This consists of a cascade of proteolytic reactions leading to the formation of membrane attack complexes on the plasma membrane of the microorganisms that cause its lysis Polysaccharides on the surface of infecting microorganism can also activate complement directly in the absence of antibody via the alternative pathway.Complement Antibodies bound to an invading microorganism activate the complement system via the classical pathway. This consists of a cascade of proteolytic reactions leading to the formation of membrane attack complexes on the plasma membrane of the microorganisms that cause its lysis. Polysaccharides on the surface of infecting microorganism can also activate complement directly in the absence of antibody via the alternative pathway