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.