Interleukin 9 (IL-9) is a γc-family cytokine that is highly produced by T-helper 9 (Th9) cells. IL-9 could promote the survival and activation of various cellular targets, including mast cells, B cells, T cells, and structural cells. Its expression is considered a hallmark of Th2-lineage cells. Primarily studied in Th2-type immunity, IL-9 was shown to be involved in asthma, allergy, and host defense against helminth infections. IL-9 also stimulates cell proliferation and prevents apoptosis. It functions through the interleukin 9 receptor (IL9R), which activates different signal transducer and activator (STAT) proteins and thus connects this cytokine to various biological processes.