Interleukin-9 (IL-9) is a type of cytokine, which are proteins that help regulate immune responses in the body. It is primarily produced by a subset of immune cells known as T-helper 9 (Th9) cells. IL-9 plays a role in promoting the growth and survival of certain immune cells, particularly mast cells, which are involved in allergic reactions and inflammation. Research has shown that IL-9 is implicated in various conditions, including asthma and other allergic diseases. It can enhance the production of other cytokines and contribute to the overall immune response, making it a target for potential therapeutic interventions in managing immune-related disorders.