Erythema multiforme: demonstration of immune complexes in the sera and skin lesions.

In twenty patients with erythema multiforme we investigated circulating immune complexes and their deposition in the skin lesions. CIq-binding activity was elevated in ten of twenty patients, and the platelet aggregation titre was high in three of twelve tested sera. Decreased levels of C3 were seen in two and of C4 in one out of eighteen patients. Direct immunofluorescence showed a deposition of C3, IgM or IgG in the blood vessel walls of the upper dermis in four of twelve patients. These findings may suggest that transient production of circulating immune complexes and their deposition play an important role in the pathogenesis of this disease.