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IL-17 is a cytokine that plays an important role in orchestrating innate immune function. In addition, IL-17 has been shown to exacerbate autoimmune diseases. CD4(+) alphabeta T cells, gammadelta T cells, and NK cells all produce IL-17. Th17 cells are a newly defined alphabeta(+) T cell lineage characterized by IL-17 production. However, gammadelta T cells are often the major source of this cytokine. Their response can be very rapid during bacterial infections and has been shown to be protective, but IL-17-producing gammadelta T cells have also been found to exacerbate collagen-induced arthritis. Interestingly, some gammadelta T cells produce IL-17 in response to IL-23 alone, even in naïve animals, suggesting they are already differentiated and may develop differently than CD4(+) alphabeta Th17 cells.