Interleukin 13 (IL-13) is a type I cytokine, and signals through type I cytokine receptors to activate JAK/STAT and IRS-1/IRS-2 pathways. IL-13 is produced by T cells and innate lymphoid cells (Pulendran and Artis) and it inhibits production of pro-inflammatory cytokines, prostaglandins, and reactive oxygen and nitrogen species by monocytes and macrophages (Hershey). Unlike human B cells, IL-13 has no effect on mouse B cell development and function. IL-13 promotes eosinophil survival, activation, and recruitment, and also activates mast cells (Hershey). IL-13 regulates gastrointestinal parasite expulsion, airway hyperresponsiveness, allergic inflammation, tissue eosinophilia, goblet cell hyperplasia, intracellular parasitism, tissue remodeling, tumor cell growth, and fibrosis. IL-13 induces hypercontractility of smooth muscles by directly acting on the muscle cells as well as the enteric nerves (Wynn).