Interleukin-25 (IL-25), also known as interleukin-17E (IL-17E), is a 17 kDa protein and member of the IL-17 cytokine family. This cytokine is secreted by a variety of cells including type 2 helper T cells (Th2), bone marrow-derived mast cells, and alveolar macrophages. On target cells, IL-25 can induce the production of several type 2 cytokines including IL-4, IL-5, and IL-13 and implicates a role for this cytokine in type 2 immune response. Studies have linked this cytokine in promoting airway hyperreactivity, limiting pro-inflammatory cytokine production and chronic intestinal inflammation, and providing protective immunity following intestinal helminth infection.