Langerhans cells as macrophages in skin and lymphoid organs.

Properties of epidermal Langerhans cell were compared with those of a number of other dendritic cells in lymphoid organs and of mononuclear phagocytes. Among the dendritic "reticulum" cells included were indeterminate cells from the epidermis, interdigitating "reticulum" cells from T-dependent areas of lymphoid tissue and thymus, follicular dendritic cells of Nossal, and the dendritic cells described by Steinman and Cohn. Interdigitating cells with typical Birbeck granules, in the thymus and in the paracortices of lymph nodes, which are morphologically indistinguishable from Langerhans cells and indeterminate dendritic cells in the epidermis, appear to belong to the same system and possibly represent a subpopulation of "macrophages." On the basis of their similarity to these other dendritic cells, we believe Langerhans cells may function in antigen presentation, lymphokine production, provision of a microenvironment for T lymphocytes, and prostaglandin secretion.

Contact hypersensitivity and Langerhans cells.

Allergens or antigens when introduced on or in the skin may become localized at the surface of and in Langerhans cells (LCs). Langerhans cells are a strategically situated cell population able to migrate into lymphatics and lymph nodes. During the course of contact allergic reactions, they are apposed to lymphocytes; some LCs are injured by interaction with lymphocytes and by antigen-antibody complexes plus complement. It is not yet clear to what extent these LCs may then release the substances they contain, such as enzymes from lysosomes, and cause further inflammatory changes. In contact dermatitis they appear to play the role of antigen presenters, and may also be target cells and inflammation-producing cells. Since in contact allergy the major antigen presentation occurs via skin, it is likely that the presence of functional LCs with intact Ia antigens is of paramount importance for induction and elicitation of this immune response.