The document summarizes the complement system, which is a group of plasma proteins that interact in a cascading fashion to protect the host against infectious agents or foreign proteins. It plays a major role in host defense and the inflammatory process. There are about 20 complement proteins that are normally inactive but can be activated through the classical, lectin, or alternative pathways. Activation of complement causes lysis of foreign cells, sensitizes cells to phagocytosis, and causes the release of inflammatory mediators. The complement system is tightly regulated by several inhibitory proteins to prevent damage to host cells.