The complement system is a set of plasma proteins that serves as an effector of several biologic functions associated with inflammation, immunoregulation, and cytotoxicity. Deficiency of complement component-2 (C2D) is the most common defect of the complement system in persons of western European descent. In type I C2 deficiency, no C2 protein is translated; in type II, there is a selective block in C2 secretion. More than half of individuals with homozygous C2 deficiency have rheumatologic disorders such as systemic lupus erythematosus, Henoch-Schonlein purpura, or polymyositis. Other individuals experience recurrent pyogenic infections, and some C2-deficient individuals are asymptomatic. [From OMIM]