The complement system is a cascade of enzyme activations that bridge the innate and accquired immune systems. Biological effects of the complement system include opsonization, lysis of foreign cells, clearance of immune complexes and apoptotic cells, activation of inflammation and augmenting the antibody response.
The complement system has three pathways based on initiation of the complement cascade: classical pathway, alternate pathway and lectin pathway. The classical pathway is triggered by the binding of C1 (a complex of complement associated proteins) to the antigen-antibody (Ag-Ab) complex. The lectin pathway is independent of Ag-Ab complex and is initiated by MBL (mannose binding lectin), a serum protein, binding to mannose groups on bacterial cell walls. The alternate pathway is activated by C3, a complement protein, binding to components of microbial cell surfaces.
The three activation pathways converge into a final common pathway when C3 convertase cleaves C3 into C3a and C3b where C3b is directly linked to opsonization and also goes on to form part of the complex that cleaves C5 into C5a and C5b. C5b with C6, C7, C8, and C9 form (C5b6789) the membrane attack complex, also known as MAC, which is inserted into the cell membrane and initiates cells lysis.