Complement deficiency is a category of primary immunodeficiency disease. The complement system comprises a complex group of proteins that play a role in host defense and inflammation. These proteins act as a cascade that involves three parallel pathways, each activated by different stimuli: the classical pathway (CP), alternative pathway (AP), and lectin pathway (LP). These three pathways converge on a terminal pathway (TP). Deficiency of any of the proteins involved in these pathways, including complement components (eg, complement component 3 [C3]) and factors (eg, factor B), may lead to recurrent infections or inappropriate immune responses. Laboratory testing for complement deficiency includes primary testing to determine which pathway is affected, followed by secondary testing to identify the deficient component or factor and reach a definitive diagnosis.
Last Update: September 2019