TNF and Airway obstruction: Harmful substances may also lead to macrophage stimulation, and along with lung epithelial cells, cytokines such as TNF (tumor necrosis factor)-a and IL (interleukin)-8 are secreted from epithelial cells, which induces the fibrosis of the bronchial tubes and alveolar macrophage destruction, resulting in irreversible airway obstruction [12].