IL-9 and Th9 in parasite immunity.

Interleukin-9 is a cytokine classically related to type 2 immune responses whose cellular identity has been recently reevaluated to identify a new specialized T helper subset called Th9 and an innate source referred as innate lymphoid cell type 2. Over the past years, IL-9 has been associated with allergic responses, tumor immunology, and autoimmunity; however, in this review, we will specifically focus on the role of IL-9 and Th9 cells in the context of parasitic infections. We will summarize and discuss all the evidence relating IL-9 expression and function in parasitic infections with a particular emphasis in helminth infections, an important health issue in developing countries; we will also provide a general description and classification of parasites, the immune response and cellular compartments activated in this context, and its implications and future directions towards a complete understanding of this interesting new T helper subset and its potential therapeutic use.