IL13 and pulmonary fibrosis: Th1 cells inhibit pulmonary fibrosis by secreting IFN-γ, Th2 cells promote fibroblast polarization and pulmonary fibrosis by secreting IL-4 and IL-13, Th17 cells promote pulmonary fibrosis by secreting IL-17, and Treg cells promote pulmonary fibrosis in the early stage and inhibit it in the late stage.