Langerhans’ cells account for 3% to 5% of the cells in the epidermis. They are dendritic cells and thus superficially resemble melanocytes. They do not, however, produce pigment, and they possess cell surface markers that identify them as being of monocyte-macrophage lineage. They originate in the bone marrow and from there move in and out of the epidermis as required by their role as the antigen-processing cells of the epidermis. After appropriate processing, Langerhans’ cells present the altered antigens to immunocompetent helper T cells, which, in turn, trigger the inflammatory reaction of allergic contact dennatitis. Langerhans’ cells are also prominently intermingled among the T cells found in other skin diseases, such as lichen planus and mycosis fungoides, what role, if any, they play in these situations is unknown. The so-called histiocytes of histiocytosis X are, in reality, Langerhans’ cells. Thus, as is true for all other cells in the skin, malignant transformation of these dendritic macrophages can occur. 
Langerhans’ cells are unusually susceptible to deactivation or destruction by ultraviolet light irradiation. This susceptibility to damage may be important in the induction of immune tolerance and in the pathogenesis of sunlight-induced cutaneous malignancy. Finally, Langerhans’ cells are easily infected with retroviruses of the human immuno deficiency virus (HIV) type. Subsequent, incremental infection of helper Tlymphocytes probably represents an important component of the pathogenic mechanisms responsible for the development of acquired immuno deficiency disease (AIDS).
Merkel’s Cells
Merkel’s cells are dendritic cells that, like melanocytes, are located in the basal layer of the epidermis. The physiologic role of Merkel’s cells is unknown, but the observation that the fine terminal filaments of cutaneous nerves preferentially end at or near the cytoplasmic membrane of these cells suggests that they may play some role in skin sensation. The origin of Merkel’s cells is unknown; they possess antigenic markers of both epidermal and neural type. Merkel’s cells may undergo uncontrolled proliferation to form rather aggressive, although rarely encountered, cutaneous malignancies.
Tags:antigen processing, cutaneous nerves, histiocytosis x, Human Anatomy and Physiology, langerhans cells, lichen planus, melanocytes, mycosis fungoides pathogenic mechanisms
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