Human interleukin 13 (IL-13) is a protein of 112 amino acids after cleavage of a 20 amino acid signal peptide. It is polymorphic with isoforms containing either Gly or Asp at position 61 and a Gln insertion at position 98. At the amino acid sequence level, mature human IL-13 is approximately 58% identical to mouse IL-13. It shows approximately 30% amino acid sequence identity with IL-4 and shares many biological properties with IL-4. IL-13 is produced primarily by activated T lymphocytes and 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. Although human or mouse IL-13 has effects similar to those of IL-4 on human B cells, mouse IL-13 has no effect on mouse B cells. Unlike IL-4, which is a growth and differentiation factor for human and mouse T cells, IL-13 has no effects on either human or mouse T cells. While human and mouse IL-13 are equally active on human cells, human IL-13 is much less active than the mouse protein on mouse cells. The gene for IL-13 has been mapped to human chromosome 5q31, in close proximity to the genes for IL-3, IL-4, IL-5 and GM-CSF. 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 γc chain.