T helper 17 cells (Th17) are a subset of pro-inflammatory T helper cells defined by their production of interleukin 17 (IL-17). They are related to T regulatory cells and the signals that cause Th17s to differentiate actually inhibit Treg differentiation. However, Th17s are developmentally distinct from Th1 and Th2 lineages. Th17 cells play an important role in maintaining mucosal barriers and contributing to pathogen clearance at mucosal surfaces; such protective and non-pathogenic Th17 cells have been termed as Treg17 cells. They have also been implicated in autoimmune and inflammatory disorders. The loss of Th17 cell populations at mucosal surfaces has been linked to chronic inflammation and microbial translocation. These regulatory Th17 cells can be generated by TGF-beta plus IL-6 in vitro. Differentiation Like conventional regulatory T cells (Treg), induction of regulatory Treg17 cells could play an important role in modulating and preventing certain autoimmune diseases. Treg17 (Regulatory Th17) cells are generated from CD4+ T cells. Transforming growth factor beta (TGF-β), interleukin 6 (IL-6), interleukin 21 (IL-21) and interleukin 23 (IL-23) contribute to Th17 formation in mice and humans. Key factors in the differentiation of Th17 cells are signal transducer and the activator of transcription 3 (Stat3) and retinoic acid receptor-related orphan receptors gamma (RORγ) and alpha (RORα). Th17 cells are differentiated when naive T cells are exposed to the cytokines mentioned above. These cytokines are produced by activated antigen presenting cells (APCs) after contact with pathogens. The Th17 cells can alter their differentiation program ultimately giving rise to either protective or pro-inflammatory pathogenic cells. The protective and non-pathogenic Th17 cells induced by IL-6 and TGF-β are termed as Treg17 cells. The pathogenic Th17 cells are induced by IL-23 and IL-1β. IL-21, produced by Th17 cells themselves, has also been shown to initiate an alternative route for the activation of Th17 populations. Both interferon gamma (IFNγ) and IL-4, the main stimulators of Th1 and Th2 differentiation, respectively, have been shown to inhibit Th17 differentiation. Similar to Th17 cells the Treg17 development depended on the transcription factor Stat3. Function Th17 cells play a role in adaptive immunity protecting the body against pathogens. However, anti-fungal immunity appears to be limited to particular sites with detrimental effects observed. Their main effector cytokines are IL-17A, IL-17F, IL-21, and IL-22, as well as granulocyte-macrophage colony-stimulating factor (GM-CSF). IL-17 family cytokines (IL-17A and IL-17F) target innate immune cells and epithelial cells, among others, to produce G-CSF and