During the routine screening of 152 patients with urticaria or angio-œdema for hypocomplementæmia, 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 hypocomplementæmia 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 hæmolytic Cl, Clq, Cls, 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.
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