Physical attraction of Th9 cells is skin deep

IL-9 was first identified as a growth factor for T cells and mast cells, and as an IL-2 cytokine family member with effects on hematopoietic progenitor cells and immune cells. The production of IL-9 by Th2 cells and by mast cells led to exploration of the role of IL-9 in allergic responses and parasite defense [see (1,2) for review]. Recent studies have resulted in identification of Th9 cells, a subset of CD4 Th cells that produce IL-9 and appear to be distinct from other CD4 T cell subsets (3,4). Thus, the identification of Th9 cells expanded the panel of CD4 Th subsets already including Th1, Th2, Th17, follicular helper T cells and regulatory T cells.