IL17F and asthma: In addition, several lines of evidence have shown that not only Th2 cell-derived cytokines but also Th17 cell-derived cytokines such as IL-17A and IL-17F are expressed in the airways in severe asthma patients, and that the levels of IL-17A and IL-17F in the airways are correlated with the severity of asthma, suggesting the involvement of Th17 cell-derived cytokines in the pathogenesis of severe asthma [4, 5].