Complement is a group of proteins that form the principal effector arm of the humoral immune system. 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. The use of recombinant DNA techniques has allowed the rapid determination of the primary structures of nearly all the known complement components, control proteins, and receptors, associated with the system, as well as their gene structure and chromosome location within the human genome. This text presents an introduction to the complement system which 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.Law, S. K. is the author of 'Complement - S. K.A. Law' with ISBN 9780199633562 and ISBN 0199633568.