Following the discovery of Th17 cells (see key facts), IL-17 has received a lot of attention among rheumatologists as a powerful proinfl ammatory cytokine and possible future therapeutic target (van den Berg and Miossec 2009, Sarkar and Fox 2010). Mouse IL-17 was cloned from activated mouse T cell hybridoma in 1993 and human IL-17 was described as a proinfl ammatory cytokine produced by human T cells in 1995. Ten years later, IL-17 was shown to be mainly produced by a new proinfl ammatory Th cell subset, Th17 cells (named after IL-17). IL-17 is thus primarily a T-cell derived cytokine in contrast to previously discussed cytokines TNF, IL-6 and IL-1 that are mainly produced by macrophages. IL-17 family consists of six ligands, IL-17A-F. IL-17 commonly refers to IL-17A which is the founding member of IL-17 family. The following paragraphs will focus on the biology of IL-17A.