Epidermotropism of atypical lymphocytes with cerebriform nuclei. Lymphocytes appear in the basal layer of the epidermis and at the dermal-epidermal junction. Some lymphocytes have a clear space (halo) around them which is a diagnostic feature of mycosis fungoides. In one area near the top of the image the lymphocytes are in contact with a collection of Langerhans cells which are antigen presenting cells in the epidermis and are thought to be involved in the pathogenesis of mycosis fungoides. They are distinguished from mycosis fungoides lymphocytes by their pale staining nuclei and abundant cytoplasm.