Cellular sources and immune functions of interleukin-9

Interleukin-9 (IL-9) has attracted renewed interest owing to the identification of its expression by multiple T helper (T(H)) cell subsets, including T(H)2 cells, T(H)9 cells, T(H)17 cells and regulatory T (T(Reg)) cells. Here, we provide a broad overview of the conditions that are required for cells to produce IL-9 and describe the cellular targets and nature of the immune responses that are induced by IL-9.