New observations in the histopathology of erythema nodosum.

Erythema nodosum presents variable clinical and histologic patterns of response in the vessels, septa, and fat lobules of the subcutaneous tissue. Acute or chronic phlebitis with hemorrhage may be commonly observed; acute panniculitis is observed in foci, but at time it may be the principal histologic feature; lymphocytic infiltration of fat lobules is often found, and lymphoid nodules are not infrequent; rarely, chronic granulomatous lesions involve septa or fat lobules; and proliferative lipocyte response may be observed. Because these variable histologic patterns may be coexistent, excision biopsy and multiple sections are necessary to recognize them. The variable histologic features correlate with the different clinical forms: acute nodular, chronic nodular, migrating plaque lesion, cellulitis, and the rate suppurative form. This variety of panniculus response in erythema nodosum can be explained on the basis of host-tissue response to a delayed hypersensitivity reaction to an antigenic stimulus.