Complement provides a critical and multifaceted defence system against infection. Following activation, complement can clear invading microorganisms by lysis or by opsonization, which promotes recognition by complement receptors on phagocytes. Complement activation products promote a local inflammatory response and are involved in the modulation of adaptive immune responses that lead to antibody production. The host has developed regulatory mechanisms for controlling complement activation and for protecting its own cells against complement attack. Some microorganisms have also evolved ways to avoid the opsonic and lytic action of complement.