IL-9 was initially identified in mice as a T cell growth factor. Human and murine IL-9 show a 56% amino acid sequence homology. IL-9 is a cytokine that acts as a regulator of a variety of hematopoietic cells, stimulates cell proliferation, and prevents apoptosis. The IL-9 receptor consists of the cytokine-specific IL-9 receptor α-chain (IL-9Rα) and the common γ-chain that is shared by IL-2, IL-4, IL-7, IL-15, and IL-21. IL-9 is expressed by activated Th9, Th17, Tregs, and mast cells and appears to play an important role in the development and function of subsets within the CD4+ T cell lineage. IL-9 also inhibits immune responses by enhancing the suppressive properties of Tregs and by recruiting immunosuppressive mast cells to sites of inflammation. Genetic studies on a mouse model of asthma demonstrated that this cytokine is a key factor in the pathogenesis of bronchial hyperresponsiveness. In this sense, it has been described that IL-9 and IL-9R expression are increased in lungs of asthmatic patients. In addition, IL-9 promotes mast cell mediated intestinal permeability and plays a role in the development of food allergies. IL-9 has also been associated with atopic dermatitis.