Interleukin 9 (IL-9) is a glycosylated protein with a molecular weight of 14 kDa containing 10 cysteine residues involved in disulfide bonding. It is a cytokine produced by T cells, especially CD4+ helper cells, and regulates a wide range of hematopoietic cells. This cytokine stimulates cell proliferation and prevents apoptosis. It acts by binding to the IL-9 receptor, which activates different signal transduction and activation (STAT) proteins and thus participates in a variety of biological processes. The gene encoding this cytokine has been identified as a candidate gene for asthma. Genetic studies of asthma in mouse models suggest that this cytokine is a determinant in the pathogenesis of bronchial hyperresponsiveness.The presence of an IL-9-mediated autocrine loop is suggestive of the presence of some malignancies such as Hodgkin’s disease.IL-9 is expressed by Lee-Schiff’s cells, Hodgkin’s lymphoma cells, and some large anaplastic lymphoma cells, whereas non-Hodgkin’s lymphoma and peripheral T-cell lymphomas do not expressed.