Flat warts undergoing involution: histopathological findings.

Patients with multiple flat warts were observed during the period of involution, shortly before regression of the warts. The histopathological process was characterized by mononuclear cell infiltration around subpapillary blood vessels, exocytosis of mononuclear cells into the epidermis, and a spectrum of degenerative epidermal changes that culminated in focal areas of necrosis within the epidermis. Lesions near the end stage of involution did not show the histopathologic features of flat warts. The mononuclear cell-associated injury to the epidermis resembles that seen in delayed cutaneous hypersensitivity responses and suggests that regression of flat warts may be due to a cell-mediated rejection reaction.