The Langerhans' cells *

The morphological, enzymatical, immunocytochemical and functional properties of Langerhans' cells are briefly reviewed. Langerhans' cells are located mainly in the squamous stratified epithelia, but are also present in the thymus and in superficial lymphnodes. At the ultrastructural level, they are characterized by unique cytoplasmic organelles, the Birbeck granules, whose function is still unknown. Langerhans' cells possess strong ATPase activity and are weakly positive for alpha‐naphtyl acetate esterase and for acid phosphatase; they are immunoreactive for CD1a (T6), class II MHG antigens and S‐100 protein. In some pathological conditions, like dermatopathic lymphadenopathy and Langerhans' cell histocytosis, Langerhans' cells also are characterized by the expression of monocyte‐macrophage antigens. Langerhans' cells act as antigen‐presenting cells to T lymphocytes; their functional capacity is strictly dependent on the levels of expression of class II MHG antigens. Langerhans' cells are of bone marrow origin and are derived from a circulating precursor which is probably related to the monocyte.