Interleukin 13 (IL-13) is a cytokine of 111 amino acids (molecular weight 15.8 kDa) whose major roles include down-modulation of macrophage activity (lowering the production of pro-inflammatory cytokines) and mediation of allergic responses. It is secreted by many cell types, but primarily by activated T-cells, in particular T-helper type 2 cells. IL-13 affects immune cells in a manner similar to IL-4 but is more associated with physiological changes induced by allergic inflammation. The effects of IL-13 are induced through a receptor that includes the alpha chain of the IL-4 receptor and at least one or two known IL-13 specific binding chains. IL-13 is the central mediator of allergic asthma, where it regulates eosinophilic inflammation, mucus secretion, and airway hyperresponsiveness.4 IL-13 has, therefore, become a therapeutic target for allergic diseases with several anti-IL-13 antibodies under evaluation as treatment for bronchial asthma. Manipulation of IL-13 effector function may also prove useful in the treatment of some cancers like B-cell chronic lymphocytic leukemia and Hodgkin’s disease, where IL-13 modulates apoptosis or tumor cell growth.