Th17 cells have crucial functions in host defense, and dysregulated Th17 responses mediate a variety of autoimmune and inflammatory conditions. Th17 cells coexpress IL-22, and its receptor is expressed on epidermal keratinocytes. IL-17 and /or IL-22 induce the production of certain cytokines, chemokines and antimicrobial peptides by keratinocytes, and its cooperation with IL-22 has been documented. Recent findings have suggested that Th17 cells profoundly participate in the pathogenesis of certain skin disorders, in particular, psoriasis. The involvement of Th17 cells has also been shown in allergen-specific immune responses. The percentage of Th17 cells is increased in peripheral blood of patients with atopic dermatitis (AD) and associated with the severity of AD. Drug eruption is another disease where circulating Th17 cells are increased.