Interleukin-13 (IL-13) was first recognized for its effects on B cells and monocytes, where it upregulated class II expression, promoted IgE class switching and inhibited inflammatory cytokine production. IL-13 is a monomeric 17 kDa immunoregulatory cytokine that plays a key role in the pathogenesis of allergy, cancer, and tissue fibrosis. Mature human IL-13 shares approximately 58% amino acid sequence identity with mouse and rat IL-13. It has become evident that IL-13 is a key mediator in the pathogenesis of allergic inflammation, and IL-13 is key proinflammatory cytokines in asthma. The cytokines interleukin IL-13 have pleiotropic effects on a variety of cell types and impact both pathologic changes and tissue remodeling.