The symphony of the ninth: the development and function of Th9 cells

CD4+ T helper cells are obligate regulators of inflammatory disease. An expanding cadre of T helper (Th) subsets, specialized for promoting particular types of inflammation, function through the secretion of a restricted set of cytokines. The latest addition to the list of subsets is the Th9 cell that secretes IL-9 as a signature cytokine and contributes to several classes of inflammatory disease. In this review we focus on recent advances in understanding the development of Th9 cells, and how Th9 cells contribute to the orchestration of disease.