The discovery of the key roles of interleukin-17A (IL-17A) and IL-17A producing cells in inflammation, autoimmune diseases and host defense has led to the experimental targeting of the IL-17A pathway in animal models of diseases as well as in clinical trials in humans. These therapeutic agents include biological products that target IL-17A and IL-23, an upstream regulator of IL-17A production. IL-17A producing T helper cells (Th17 cells) are a distinct lineage from the Th1 and Th2 CD4+ lineages and have been suggested to represent a good drug target in certain inflammatory conditions. Targeting IL-17A has been proven to be a good approach as anti-IL-17A is FDA approved for the treatment of psoriasis in 2015. In host defense, IL-17A has been shown to be mostly beneficial against infection caused by extracellular bacteria and fungi. This review will overview the discovery of IL-17A, the receptors used by this cytokine and its role in mucosal immunity and inflammation. (C) 2017 Elsevier B.V. All rights reserved.