Interleukin 13 (IL-13) is a single-chain glycosylated polypeptide, which belongs to the IL-13/IL-4 family. IL-13 protein is secreted by many cell types, but especially by T helper type 2 (Th2) cells. IL-13 exerts its effects through a multi-subunit receptor comprising the alpha chain of the IL-4 receptor (IL-4Rα) and at least one of two known IL-13-specific binding chains (IL-13 Rα1 and IL-13 Rα2). As a cytokine, IL-13 protein is critical in regulating inflammatory, immune responses and diseases. In addition, it inhibits the production of pro-inflammatory cytokines and chemokines, and thus down-regulates macrophage activity. IL-13 protein and antibody is more importantly implicated as a central mediator of immunoregulatory processes in various cell types.