Molecular Mechanisms Underlying the Actions of the Complement System

Complement is crucial to the immune system. It is initiated by the classical, lectin, and alternative pathways. Each pathway involves initial binding to a foreign surface, triggering the activation of proteases. These cleave and combine with further molecules of the system to effect amplification and assembly of structures such as the membrane-attack complex, which kills the targeted pathogen or altered cell. Targeted cells are also tagged for removal by immune system cells attracted to the site of complement activation by peptides released. The system is highly regulated, but is also involved in many inflammatory diseases.