Reduction of the C 2 fraction of complement : a family study

Background: Familial hereditary angioedema due to genetic changes affecting the synthesis of complement components other than C1 inhibitor is an infrequent phenomenon; its association to urticaria is even less frequent. After the detection of low C2 and C4 levels in a 22-year-old woman consulting because of facial angioedema, erythrodermia and urticaria, a familial study was indicated. Methods: The complement fractions were analysed in all members of the family unit. The C1 inhibitor, C3 and C4 fractions were quantified by nephelometry, and the C2 fraction by radial immunodiffusion. The functional C1 inhibitor activity (chromogenic substrate technique) and the CH50 were assessed by spectrophotometry. Results: Low C2 levels were detected in all the members of the family unit. In both parents, the change was selective for that fraction. The consulting patient (index case) evidenced a partial deficiency of the C2 and C4 fractions, and her three sisters had low levels of C2, C3 and C4. No quantitative or functional derangement of the C1 inhibitor or of CH50 was detected in any of the members of the family. The C2 quantitation in the consulting patient's spouse yielded normal results. Conclusions: The studied case might be one of heterozygosity with expression of a null gene, or one of an atypical regulation disorder.