Interleukin-9 (IL-9) is a secreted protein that belongs to the IL-7/IL-9 family. IL-9 supports IL-2 independent and IL-4 independent growth of helper T-cells. IL-9 stimulates cell proliferation and prevents apoptosis. It functions through the IL-9 receptor (IL-9R), which activates different signal transducer and activator (STAT) proteins and thus connects this cytokine to various biological processes. IL-9 has been identified as a candidate gene for asthma. IL-9 is a determining factor in the pathogenesis of bronchial hyperresponsiveness.