Interleukin 9 (IL-9) is a multifunctional cytokine produced by activated TH2 clones in vitro and during TH2-like T cell responses in vivo. Although IL-9 was initially described as a T cell growth factor, its role in T cell responses is still unclear. While freshly isolated normal T cells do not respond to IL-9, this cytokine induces the proliferation of murine T cell lymphomas in vitro, and in vivo overexpression of IL-9 results in the development of thymic lymphomas. In the human, the existence of an IL-9-mediated autocrine loop has been suggested for some malignancies such as Hodgkin's disease. Various observations indicate that IL-9 is actively involved in mast cell responses by inducing the proliferation and differentiation of these cells. In addition, both genetic and experimental evidence points to the implication of IL-9 in the pathogenesis of asthma. Other potential biological targets for IL-9 include В lymphocytes, eosinophils, and hematopoietic progenitors, for which higher responses were observed with fetal or transformed cells as compared with normal adult progenitors. The IL-9 receptor is a member of the hematopoietin receptor superfamily and interacts with the 7 chain of the IL-2 receptor for signaling.