Protein contact dermatitis (PCD) is caused by proteins. The patients may show positive immediate or delayed reactions in skin prick, scratch or patch test, or the skin tests may remain negative. Specific IgE is obviously crucial in most reactions, however, the exact mechanisms remains unclear, specifically in the delayed reactions. To address these issues, we investigated mice sensitized by papain enzyme as a protein allergen.Mice were sensitized by 24hrs exposure to papain twice a week for one month. The draining lymph node cells were then isolated and re-stimulated by papain. ELISA and FACS analyses of the cytokines synthesized in lymph node cells revealed that CD4+ IL4+ T (Th2) and CD4+ IL17+ T (Th17) cells were differentiated in the draining lymph nodes of the mice sensitized by papain. However, CD4+ IFN-gamma+ T cells (Th1) were not detected.The sensitized mice were elicited by applying papain onto the ear to address the role of Th2 and Th17 cells differentiated in the papain-sensitized mice, and the allergic reactions in the ear were examined. Real-time PCR analyses revealed that IL-17 mRNA of Th17 marker was up-regulated in a time-dependent manner; however, IL-4 mRNA of Th2 maker was not detected at any time points. These phenomena were confirmed by immunofluorescent microscopy using a specific antibody for IL-17, suggesting that Th17, but not Th2, migrated into the ear after the elicitation. Additionally, the papain-sensitized mice possessed papain-specific IgE, and mast cell degranulation was observed after the elicitation.These findings reveal for the first time that the sensitization of papain enzyme differentiates nau00efve T cells into not only Th2 but also Th17 cells as effector T cells contributing to the allergic reactions. The generated Th2 produces IL-4 and is involved in IgE syntheses in the draining lymph nodes. Conversely, the generated Th17 cells migrate to the skin and produce IL-17 after elicitation, which may involve in the delayed reactions.