Cutaneous immunofluorescence study of erythema multiforme: correlation with light microscopic patterns and etiologic agents.

Direct immunofluorescence microscopy was positive in 88% of forty-one skin biopsy specimens from thirty-four patients with the clinical diagnosis of erythema multiforme. The most common finding, present in 67% of positive specimens, was the cytoid body, or fluorescent keratinocyte, which stained most often with IgM (homogeneously) or with C3 (speckled). Other findings included basement membrane zone (BMZ) fluorescence, primarily with fibrinogen and C3, and vascular fluorescence, most commonly C3 in a granular pattern. Correlation of direct immunofluorescent and light microscopic findings revealed that (1) the fluorescent keratinocyte was prevalent only in epidermal and mixed patterns, correlating with the eosinophilic necrotic keratinocyte by light microscopy, and (2) vascular fluorescence was most prominent in dermal forms. Herpes simplex-associated erythema multiforme showed exclusively a mixed histologic pattern, whereas the drug-related form was primarily epidermal.