Interleukin 17A (IL-17 or IL-17A), is the founding member of a group of cytokines called the IL-17 family. Known as CTLA8 in rodents, IL-17 shows high homology to viral IL-17 encoded by an open reading frame of the T-lymphotropic rhadinovirus Herpesvirus saimiri. The most notable role of IL-17 is its involvement in inducing and mediating proinflammatory responses. IL-17 is commonly associated with allergic responses.

Interleukin 17 is a pro-inflammatory cytokine produced by T-helper cells and is induced by IL-23. To elicit its functions, IL-17 binds to a type I cell surface receptor called IL-17R of which there are at least three variants IL17RA, IL17RB, and IL17RC.[3] IL-17 acts as a potent mediator in delayed-type reactions by increasing chemokine production in various tissues. Signaling from IL-17 recruits monocytes and neutrophils to the site of inflammation in response to invasion by pathogens, similar to Interferon gamma. In promoting inflammation, IL-17 has been demonstrated to act synergistically with tumor necrosis factor and interleukin-1.