Interleukin 13 (IL-13) is produced primarily by activated T lymphocytes. It shows approximately 30% amino acid (aa) sequence identity with IL-4 and shares many biological properties with IL-4. IL-13 has multiple effects on the differentiation and functions of monocytes/macrophages. It can suppress the cytotoxic functions of monocytes/macrophages and the production of proinflammatory cytokines by monocytes/macrophages. The biological effects of IL-13 are mediated by specific high-affinity cell surface receptor complexes. The functional IL-13 receptor complex has been shown to consist of the low-affinity IL-13 receptor α chain and the IL-4 receptor α subunit (IL-4 Rα). This IL-13 receptor complex may also serve as an alternate high-affinity IL-4 receptor complex in IL-4 responsive cells that lack the γ chain.