IL-9 is a type of cytokine, which is a protein that helps regulate the immune system. It is primarily produced by a subset of immune cells known as T-helper 9 cells. IL-9 plays a role in promoting the growth and activation of certain immune cells, particularly mast cells, which are involved in allergic responses and inflammation. Research has shown that IL-9 is linked to various allergic conditions, such as asthma and hay fever. It can also influence the development of certain autoimmune diseases. Understanding IL-9 is important for developing targeted therapies to manage these conditions effectively.