Recently, it has been demonstrated that immunity to infectious agents is composed of innate immunity and acquired immunity, and that dendritic cells (DC) and macrophages, both of which are the participants in the innate immunity, play a crucial role in acquired immune responses, via their expression of several costimulatory molecules and production of cytokines. It is clear that the immune system responds not only to infectious organisms but also to simple chemicals. Allergic contact hypersensitivity reaction is a good example of the immune response to simple chemicals. In contrast to the immunity to microorganisms, however, the role of the innate immune system in responses to simple chemicals still remains unclear. This paper demonstrates that the activation and apoptosis of DC are directly induced by certain simple chemicals, and we suggest that DC, as cells involved in the innate immune system, play a crucial role in the immunity to simple chemicals.