Complement system

The complement system is a cascade of plasma proteins that forms one of the oldest branches of the immune system, predating the adaptive immune system by hundreds of millions of years. When activated — typically by antibody-antigen complexes, pathogen surfaces, or acute-phase proteins — complement proteins cleave each other in an amplifying enzymatic cascade that produces three principal outcomes: opsonization (coating pathogens for phagocytosis), inflammation (recruiting immune cells to the site of infection), and direct lysis through the membrane attack complex.

The complement system is not a separate immune subsystem but a distributed amplification network that integrates innate recognition with adaptive effector function. Antibodies that cannot neutralize a pathogen directly can still activate complement, turning a weak binding event into a powerful inflammatory signal. This bridging function — connecting the specificity of adaptive immunity to the rapid response of innate immunity — makes complement an essential node in the immune network's topology.