Complement is a group of serum proteins that play an important role in defence against infection. The main functions of complement are:
- Opsonisation, to enhance the uptake of microorganisms by phagocytes
- Inflammation, to facilitate the entry of cells at a site of infection
- Cell lysis of certain microorganisms
- Clearance of immune complexes (complexes consisting of antibody bound to antigen)
In order to carry out these functions complement must be activated, a process that involves proteolytic cleavage of certain components of the complement system to generate active molecular species. Complement may be activated in one of three ways: directly by certain microbes (the alternative pathway); indirectly in the presence of antibody (classical pathway); or indirectly by carobohydrate binding molecules (the lectin pathway). Different complement proteins are involved in the initial events in these pathways but they converge to give rise to an enzyme that breaks down the complement protein C3, which is central to the complement system.
Complement is tightly regulated by a number of soluble and cell surface inhibitory molecules, that keep the system in check and ensure that it is not activated inappropriately, which could have pathologiccal consequences.