Complement C4 (C4) plays a central role in both the classical and lectin pathways of complement activation, with levels in human plasma ~0.4 mg/ml. C4 circulates in human plasma as a heterotrimer of 205,000 Da in plasma. The three disulfide-linked chains have molecular weights of 93 (alpha), 75 (beta), and 32 (gamma) kDa. These are readily processed to create C4a, C4b or C4c depending on the processed state. Complement C4c is a degradation product of C4b which is cleaved by C4b/C3b inactivator to yield C4d and C4c. Complement C4c interacts with C1 and C2 to form C3 convertase of the classic activation pathway. Systemic lupus erythematosus (SLE) is often associated with congenital C4 deficiency. Reduced levels of components of the classic pa