Langerhans cells mainly present in the normal epidermis and are thought to be identical to the proliferating cells in the lesions of histiocytosis-X. The Langerhans cell is positive not only for ATPase, esterase, and acid phosphatase, but also for S-100 protein. Since these findings are similar but distinct from those of monocytes and macrophages, the Langerhans cell may be a cell line independent of the monocyte-macrophage system, designated as T-zone histiocytes. Immunologically, the Langerhans cell has Fc and C3 receptors and Ia-like antigen. Thus, the Langerhans cell may be a member of the immune network, as antigen-presenting cells, in the epidermis. One hypothesis is that Langerhans cells activated by external stimuli induce a local immune reaction by activation and proliferation of T-cells, with the interaction of keratinocytes and macrophages. Because of cytochemical and immunologic similarities between Langerhans cells and histiocytosis-X cells, histiocytosis-X may be a proliferative disorder of immature or neoplastic Langerhans cells.