RESUMO
The complement system is a fundamental component of the host immune response. In addition to its effector activity against pathogens, it possesses functions such as opsonization and phagocytosis, removal of immune complexes and activation of the inflammatory process. The knowledge of the complement system is important in the investigation of numerous diseases that can be observed in cases of deficiencies in cascade proteins, their receptors, or regulatory proteins. Clinical and experimental evidence demonstrate the association between the complement system and several inflammatory conditions, as well as a greater susceptibility to infection among patients with complement system dysfunction. Thus, the purpose of this paper is to describe the three complement system pathways - the activation and effector mechanisms and their biochemical characteristics - and correlate them to certain clinical conditions.