Although once thought to play a purely structural role, there is increasing evidence that keratinocytes are actively involved in epidermal immune responses, including allergic contact dermatitis (ACD). In vitro studies demonstrate that both urushiol and nickel sulphate induce cytokine production in cultured keratinocytes including molecules responsible for endothelial cell activation and lymphocyte chemotaxis and adhesion. In vivo, these same molecules are expressed in experimentally induced patch test reactions to a variety of allergens. Furthermore, such expression precedes the onset of the inflammatory phase of ACD. Taken together, these studies suggest a role complementary to that of Langerhans cells for keratinocytes in the initiation and propagation of ACD.