IL17A and chronic rhinosinusitis: Th2, Th1, and Th17 cells have been implicated in chronic diseases of the paranasal sinuses, that is, Th1-related cytokine IFN-γ in chronic rhinosinusitis without nasal polyps (CRSsNP), Th2-related cytokines IL-4 and IL-5 in CRS with nasal polyps (CRSwNP), and recently Th17-cell-related IL-17 in Asian nasal polyps and polyps in cystic fibrosis patients [22, 23].