Selective depression of the C4 component of complement: evidence for an association with genetic deficiency of C4 and dermatologic diseases.

Thirty-four patients with selective low serum C4 levels were found among 1,731 patients studied in a hospital-based routine clinical immunology laboratory. A high incidence of dermatologic diseases, particularly discoid lupus erythematosus and angioedema/urticaria, was present among patients with selective low C4. Although evidence for activation of C4 was present in patients' sera, studies to determine the presence of known activation mechanisms of complement were negative. Genetic C4 typing in four pedigrees showed that all propositi carried the null allele B*QO. It is postulated that the selective C4 depression in these patients is related to the skin lesions and this partial genetic deficiency.