T-helper 9 cells are a type of immune cell that play a crucial role in the body's defense against infections and diseases. They are a subset of T-helper cells, which are essential for coordinating the immune response. T-helper 9 cells are characterized by their production of specific cytokines, particularly interleukin-9, which helps regulate inflammation and support the activity of other immune cells. These cells are involved in various immune responses, including those against parasites and allergens. Research has shown that T-helper 9 cells can also contribute to certain autoimmune diseases and allergic reactions, highlighting their importance in both protective immunity and potential pathological conditions.