In allergic (contact) dermatitis and certain cases of drug idiosyncrasy, the epithelium of the skin is probably sensitized. The hypersensitiveness of the epithelium develops by contact with the injurious agent in the same manner as the usual forms of hypersensitiveness.
The tuberculin type of skin reactions present many characteristics that suggest the direct participation of the cutaneous epithelium in the reaction. The characteristic necrosis in the reactions is often limited to the epithelium and a thin layer of connective tissue below it. The epithelium is involved early before the macroscopic reaction is strong. The epithelium is often infiltrated with polymorphonuclear leucocytes even in slight tuberculin reactions. The occurrence of necrosis is in no direct connection with the size of the reaction or with the general sensitiveness.