IL17A and chronic obstructive pulmonary disease: Biological factors acting as antigens can differentially induce Th1, Th2, and Th17 cell immune responses, leading to the release of IFN-gamma, IL-5/IL-13, and IL-17; subsequently, these immune response factors lead to the activation of the mononuclear, eosinophilic, and neutrophilic pathways, respectively, and contribute to the development of COPD, asthma, and lung cancer (Fig. 1b).