Partial deficiency of the fourth component of human complement (C4) and autoantibody directed against C4 in a patient with SLE.

The finding of dramatically depressed levels of C4 in a 17-year old patient with severe systemic lupus erythematosus (SLE) prompted a genetic study of her family. This study revealed the existence of a partial C4 deficiency; we found the presence of a C4A3,C4BQo haplotype which was transmitted by the mother to each of her children. This patient possessed, in her serum, an autoantibody with anti-C4 specificity. The immunochemical characterization of this autoantibody revealed that it was IgM and belonged to the immunoconglutinin family. We have studied the effects of this autoantibody on the formation and dissociation kinetics of classical C3-convertase.