In this diagram, patterns of epidermal reactions are represented. A common primary patternis characterized by elongation of rete ridges and hyperplasia of the basal unit of the epidermis; the patterns are psoriasiform. Psoriasis is the prototype but such a pattern may also be encountered in early stages of some lichen planus-like reactions. On the other hand, some lesions which are basically lichenoid may evolve in stages and the primary stage oftenhas psoriasiform qualities. Psoriasiform patterns are expressive of immunostimulation with expansion of the domain of the basal unit of the epidermis. In stages. the psoriasiform pattern may give way to immunolytic patterns in which lymphocytes and histiocytes among basal keratinocytes produce death and lysis of neighboring keratinocytes; lytic defects are produced in the basal unit and the superficial unit; then, the superificial unit undergoes compensatory hyperplasia and hypertrophy. The combination of features characterize the lichenoid reaction (particularly, the pattern of the established phase) and are basic to the concept of pathologic apoptosis. The pattern is that of an established lichenoid reaction. For some variants in the stage of resolution, the defect in the epidermal domain is inlaid with firbous tissue and the basement membrane is “duplicated” in the new formed fibrous tissue. The resultant pattern qualifies as a senescent lichenoid reaction. These patterns are commonly encountered in melanocytic neoplasia and tor some examples, the changes may be so extensive that the underlying neoplasm is masked by the reaction. In the diagram, apoptotic cells, lymphocytes and histocytes all are represented in the lytic defect of the established lichenoid reaction and in the fibrous inlay of the senescent lichenoid reaction.