Moreover, contrary to the traditional belief that IL-17 only originates from Th17 cells, it is now recognized that IL-17 is also generated by IL-17-producing Th2 cells [36], with a high prevalence of these dual-function cells being detected in both the blood and bronchoalveolar lavage fluid of individuals with severe steroid-resistant asthma [37]. This evidence concerns the gene IL17A and asthma.