TNF and chronic obstructive pulmonary disease: When COPD patients’ respiratory tract is infected by viruses or bacteria, some antigen components or metabolites in viruses or bacteria such as LPS, endotoxin can activate alveolar macrophages to produce TNF-α, IL-2 and other inflammatory factors, and these inflammatory factors in turn can promote alveolar macrophages or bronchial epithelial cells to produce IL-6.