Interleukin 13 (IL-13) is an immunoregulatory cytokine produced primarily by activated Th2 cells. This cytokine is involved in several stages of B-cell maturation and differentiation. IL-13 up-regulates CD23 and MHC class II expression, and promotes IgE isotype switching of B cells. IL-13 inhibits the production of a series of cytokines like IL-1, IL-6, TNF-alpha, and IL-8 by activated human monocytes. IL-13 induces IFN-gamma production by NK cells. IL-13 is thought to be important cytokine in the pathogenesis of asthma, and more recently has been shown to play a pivotal role in a number of fibrotic diseases including hepatic and pulmonary fibrosis, and nodular sclerosing Hodgkin’s disease.