The skin is a barrier site that is exposed to a wide variety of potential pathogens. As in other organs, pathogens that invade the skin are recognized by pattern-recognition receptors (PRRs). Recently, it has been recognized that PRRs are also engaged by chemical contact allergens and, in susceptible individuals, this elicits an inappropriate immune response that results in allergic contact dermatitis. In this Review, we focus on how contact allergens promote inflammation by activating the innate immune system. We also examine how innate immune cells in the skin, including mast cells and dendritic cells, cooperate with each other and with T cells and keratinocytes to initiate and drive early responses to contact allergens.
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9. Landsteiner, K. & Jacobs, J. Studies on the sensitization of animals with simple chemical compounds. III. Anaphylaxis induced by arsphenamine. J. Exp. Med. 64, 717–721 (1936). This article first proposed the idea that the formation of the hapten–self complex is a crucial early event in allergic contact dermatitis. Seventy-five years later, this hypothesis has been supported by the work of many investigators, and remains a core concept of allergic contact dermatitis.