The existence of the complement system was first recognized at the end of the 19th century by the ability of serum to enhance the destruction of bacteria and the requirement of a heat-labile serum factor necessary for the lysis of red cells. Since then extensive research by many groups of workers led to a detailed understanding of the structure, activation sequence and biological importance of this system. It consists of a group of plasma proteins whose sequential interaction may be initiated by immunoglobulins, microbial products and proteolytic enzymes. Activation is achieved by the generation and assembly of highly specific enzymes and is accompanied by limited proteolytic cleavage; this leads to the release of biologically active peptides and the formation of large multimolecular complexes. The complement system may be divided into four functional sections, two pathways for activation (the classical pathway and the alternative pathway), an amplification mechanism for augmenting the