MMP9 and chronic obstructive pulmonary disease: Their activation can release specific inflammatory mediators, leading to irreversible airway damage, such as neutrophil elastase, matrix metalloproteinase-9 (MMP-9), tissue protease G, Matrix metalloproteinases-48 (MMP-48), and myeloperoxidase (MPO), which can promote the pathological and physiological mechanisms of COPD (22).