Interleukin 13 (IL-13) is produced predominantly by T helper 2 cells, but also by mast cells and NK cells. IL-13 is related to IL-4 and maps to a gene cluster that includes IL-4. IL-5. IL-3. and GM-CSF. The biological functions of IL-13 include the regulation of IgE secretion by В cells, the modulation of TH2 cell development, and the suppression of inflammatory responses due to regulation of macrophage function. Significantly, recent studies have indicated that IL-13 plays a central part in TH2 responses, mediating key roles in the expulsion of gastrointestinal parasites and in the development of asthma and allergy.