The complement system is a group of proteins in the blood that fights infection. This examination of the system discusses its various families of proteins and describes how they interact with each other to mediate the different functions of host defence.

The complement system is a group of proteins in the blood that fights infection. Today, more than 30 proteins have been shown to be closely involved with the complement system. By a series of specific activation steps, which are triggered by the presence of foreign entities, these proteins mediate a range of activities such as the initiation of inflammation, clearance of immune complexes, and regulation of the immune response. This introduction to the complement system covers the basic biochemistry and activation pathway, and discusses the different families of proteins in the system and how they interact with each other to mediate the various functions in host defence.