C4 binding protein deficiency in a patient with atypical Behçet's disease.

Primary deficiency of the C4 binding protein (C4bp) was present in a patient with disease clinically resembling Behçet's disease. Her father and her sister were also deficient. This protein, as a cofactor for factor I, interferes with the assembly of, and accelerates the decay of, the classical C3 convertase. Thus, the deficiency favours C3 conversion by classical pathway activation. In addition to genital and oral ulceration, cutaneous vasculitis and synovitis, our patient had relapses complicated by angioedema, atypical for Behçet's disease. It is not clear whether her total disease, or only the complicating angioedema, was a consequence of the C4bp deficiency.