Interleukin-17 (IL-17) is a family of 6 closely related cytokines, designated IL-17A-F, that play a central role in mediating inflammation, autoimmunity, and host defense. IL-17 is mainly secreted by a specific subset of T helper cells known as TH17 cells (for an overview see Fig 1). The IL-17 cytokines mediate their biological functions via surface receptors on target cells. IL-17A binds to IL-17 receptor A (IL-17RA), which stimulates the production of other pro-inflammatory cytokines including IL-6 and IL-8. IL-17A and its receptor play a pathogenic role in many inflammatory and autoimmune diseases such as rheumatoid arthritis. IL-17 signaling is also involved in mucosal immunity and host defense against extracellular bacterial and fungal infections (Staph, Candida, Pneumonia, etc.).