Necrotizing vasculitis: a circulating immune complex producing inflammatory skin lesions.

A patient with acute necrotizing vasculitis is described in whom tests for circulating immune complexes were negative. The patient's serum injected intradermally produced lesions which closely resembled those occurring spontaneously. When the serum was fractionated, the skin-reactive fraction was found to be associated with IgG, but was of a higher molecular weight than normal IgG. Immunofluorescent studies showed that lesions induced by this active fraction contained IgG and complement in the epidermal basement membrane zone and within the small dermal vessels. We conclude that an immune complex of relatively low molecular weight is present in the active fractions, and capable of initiating the lesions of acute necrotizing vasculitis.