The hypothesis of helper T(T h)1/T h2 cytokine balance proposed by Mosmann and Coffman is often invoked to explain the development of inflammatory diseases, including inflammatory bowel diseases (IBD). Recently, however, a newly identified class of T h cells - T h17 cells, which produce T h17 family cytokines - has been recognized as an essential subpopulation in the development of almost all kinds of human and animal inflammatory diseases, rather than T h1 and T h2 cells. A representative T h17 family cytokine, interleukin (IL)-17A, is produced by not only T h17 cells, but also by other types of cells, such as T-cell receptor γδ T cells, natural killer (NK) T cells, NK cells, myeloid cells, and innate lymphoid cells, which may also be critically involved in the initiation and persistence of IBD. Here we review recent advances in the study of such IL-17A-producing cells in the pathogenesis of IBD.
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