Langerhans cells are a dendritic cell type characteristic of the epidermis. Since mast cells secrete molecules potentially influent on dendritic cell differentiation, we have addressed the degree of proximity between these two cell types in biopsies of skin diseases characterized by massive influx of dendritic cell precursors. By fluorescence microscopy, avidin labeled mast cells were found in contact with CD1a+ dendritic cells. By electron microscopy, contacts between mast cells and blunty dendritic cells were found in areas corresponding to those where CD1a+ cells were localized by immunohistochemistry. We propose that mast cells can induce the differentia­ tion of precursors into Langerhans cells through both the release of short range acting soluble factors and contact­mediating plasma membrane molecules
