The complement system is the complement system is a set of proteins involved in innate but not acquired immunity a set of proteins that act individually to attack and lyse microbes a group of antimicrobial proteins that act together in a cascade fashion a group of proteins that includes interferons and interleukins

I think that the complement system is a group of antimicrobial proteins that act together in a cascade fashion.  Complement system is a part of the immune system, it has plasma proteins that can be activated directly by pathogens or indirectly by pathogen-bound antibody, leading to a cascade of reactions that occurs on the surface of pathogens and generates the active components that results to various effector functions.