C1-bypass complement-activation pathway in patients with chronic urticaria and angio-oedema.

During the routine screening of 152 patients with urticaria or angio-oedema for hypocomplementaemia, 4 patients were found to have low serum levels of the third component of complement (C). These patients were noteworthy and differed from previous reports of patients with urticaria-like skin lesions and hypocomplementaemia because of the absence of immune-complex disease. In addition to the low C3, 2 of these patients were unique on the basis of low serum levels of haemolytic C1, C1q, C1s, and properdin factor B, but normal concentrations of C4 and C2. These C abnormalities may reflect a new clinical entity, and these cases form the first description in man of the C1-bypass complement-activation pathway.