Rat IL-13 was initially cloned from the rat renal cortex RNA after the induction of anti-glomerular basement membrane antibody-induced glomerulonephritis. Rat IL-13 respectively shows 63% and 79% homology with the human and mouse proteins at the amino acid level. IL-13 is an immunoregulatory cytokine secreted predominantly by activated Th2 cells, and is a key mediator in the pathogenesis of allergic inflammation. IL-13 shares many functional properties with IL-4, and they share a common receptor subunit: the alpha subunit of the IL-4 receptor (IL-4Rα). IL-13 mediates its effects by interacting with a complex receptor system comprised of IL-4Rα and two IL-13 binding proteins: IL-13Rα1 and IL-13Rα2. The ligation of the IL-13 receptor complex results in signaling via the insulin receptor substrate (IRS)-1 and 2 and STAT-6 pathways. IL-13, like IL-4, is a cytokine produced by Th2 cells in response to the signaling through the T cell antigen receptor and by mast cells and basophils upon cross-linkage of the high-affinity receptor for immunoglobulin E (IgE). IL-13 has been implicated in airway hypersensitivity and mucus hypersecretion, chronic obstructive pulmonary disease (COPD), inflammatory bowel disease, and parasitic nematode expulsion.