Complement component C4A and C4B levels in systemic lupus erythematosus: Quantitation in relation to C4 null status and disease activity

Immunoassays using C4 monoclonal antibodies were developed to quantitate C4A, C4B and total C4 in the serum of healthy controls and patients with systemic lupus erythematosus (SLE). Mean C4A or C4B levels were reduced when a single C4A * Q0 or C4B * Q0 gene was present; however, total C4 levels showed considerable overlap with and did not differ significantly from the non-C4 null groups in either patients or controls. Black patients with SLE without active disease, as well as black controls, had higher levels of C4B and consequently total C4 than whites