[The Langerhans cell--its macrophages-analogous function in the triggering of the allergic contact eczema (author's transl)].

1. Langerhans cells represent specific granula-containing dendritic cells which do not have desmosomes, tonofilaments and (pre)melanosomes and which therefore appear as clear cells in the epithelial tissue. 2. They occur in the squamous epithelium, and also in the corium, lymph node and thymus. They account for 1-1,7% of the whole volumen of the epidermis. 3. They do not represent effete melanocytes, but they originate from the mesenchym. Their migration from the dermis into the epidermis, their identity with histiocytosis X cells, their surface receptors as well as function underline the mesenchymal origin. 4. Probably the Langerhans cells regulate the mitosis and differentiation of the keratinocytes. 5. The antigen-presenting and lymphocyte stimulating functions of Langerhans cells as effector cells in allergic contact eczema are proved. They are able to phagocytize antigens (haptens), to apposite lymphozytes, to proliferate after the challenge by antigens as well as to migrate through the lymph vessels into the regional lymph nodes. 6. At their surface they bear receptors for Fc-IgG and C3 as well as Ia-antigens as immune response genes of the major histocompatibility complex (MHC). 7. With regard to point 5 and 6 they have the same properties as the macrophages. 8. Langerhans cells are damaged and destroyed, respectively, by immune complexes after activation of the complement pathway as well as by killer T-lymphocytes. Thus they are regulated as target cells by humoral and cellular mechanisms. 9. The allergic inflammatory reaction is triggered by mediator substances (of lysosomal origin?) which are liberated by the destruction of the Langerhans cells as well as by lymphokines. The preferently suprabasal occurrence of the Langerhans cells explains the early beginning of the edema and spongiosis as well as their localization in the lower layers of the epidermis. 10. The macrophages-analogous and antigen-presenting functions, which are genetically regulated are essential for the sensitization in allergic contact eczema. The ability to stimulate allogenic T-Lymphocytes in the mixed lymphocyte reaction might be of particular importance for the skin transplantation.