Further mechanistic studies showed that Th9 cells can promote pulmonary fibrosis in the following two aspects: on the one hand, Th9 cells promote fibroblast differentiation, activation, and collagen secretion by secreting IL-9; on the other hand, Th9 cells promote the differentiation of Th0 cells to Th2 cells by secreting IL-4, which accelerates the Th1/Th2 imbalance, and ultimately promotes the formation of pulmonary fibrosis (27). The gene discussed is IL4; the disease is pulmonary fibrosis.