Complement proteins constitute a serum enzyme system that aids to mediate inflammation. Complement components are triggered into an activated form by immunologic events such as interaction with immune complexes. Complement components are identified by numbers (C1, C2, etc.). Genetic deficiencies of C1q, C2, and C4, although rare, are most commonly associated with SLE. A CH50 test evaluates the entire complement system. The most commonly measured complement components are the serum level of C3 and C4.