[Outline of the pathology of the Langerhans cell system]

About 100 biopsy specimens, most of them from the skin, were used for comparisons between high alpha-D-mannosidase activity and the presence of Langerhans' cells identified according to the electronoptic picture. Agreement between the two was absolute so that alpha-D-mannosidase could be rated as a reliable index of Langerhans' cells not only in histiocytosis X but also in a number of non-tumour and other cancerous processes in the skin. -- Contact couples (apposition) of Langerhans' cells and lymphocytes were often found in as well as outside the epidermis in mycosis fungoides just as in skin allergoes of the contact dermatitis type (less so in experimental alergic dermatitis in the rat, guinea-pig, and rabbit caused by dinitrochlorobenzene). However, specific examinations of a number of electronograms from skin biopsies fairly often revealed such couples also in seborrhoeic keratosis, in basaliomas, and in other processes. -- Despite the above mentioned dispersion variance of the results obtained, Langerhans' cells may yet be found to represent a first link in a chain of immunity responses to exogenous antigens connected with ecotaxis and leading up to a picture of skin allergoses, dermatopathic reticulosis and mycosis fungoides.