EGF and chronic obstructive pulmonary disease: Activated neutrophils secrete TNF-α, which up-regulates the EGFR expression in airway epithelial cells and directly stimulates MUC synthesis.89 The expression of EGFR was higher in COPD patients than in smokers with normal lung function, which indicated that COPD was related to the overexpression of EGFR.90 EGFR and its ligand EGF binding are the main causes of squamous cell metaplasia, and the growth of epithelial cells is most significant in smokers and COPD.91 These indicate that EGFR levels in the small airways of COPD patients were associated with decrease in airway functionality.