Light and electron microscopical studies of erythema multiforme showed two types of tissue changes with the bullae forming at a different location in each type. (a) Erythematous papular lesions revealed primarily dermal edema resulting in intradermal bulla formation, with the basal lamina at the top of the blister. (b) "Target" lesions showed primarily epidermal damage resulting in bulla formation at the dermal-epidermal junction, with the basal lamina, if preserved, at the floor of the bulla. Vascular changes were encountered in both types. These findings show that analogous to the multiform clinical appearance of erythema multiforme, the type of bulla that develops in a given case depends on whether the dermis or the epidermis is primarily involved.
Constantin E. Orfanos, Gundula Schaumburg-Lever, Walter F. Lever. Dermal and Epidermal Types of Erythema MultiformeA Histopathologic Study of 24 Cases. Arch Dermatol. 1974;109(5):682–688. doi:10.1001/archderm.1974.01630050028006