The complement system is a complex system of proteins that play a role in host defense and inflammation response by acting as a cascade involving three pathways – classical (CP), alternative (AP), and lectin (LP). Each pathway is initiated by distinct mechanisms. Deficiency of any of the proteins in these pathways may lead to recurrent infections (with encapsulated organisms, in particular) or inappropriate inflammatory responses. Inherited deficiencies are uncommon, while acquired deficiencies, due to a variety of etiologies, are much more common.
Last Update: February 2019