It has been identified that neutrophil accumulation in the lung was a major cause of acute lung injury in many respiratory diseases (e.g., COPD, acute respiratory distress syndrome, and cystic fibrosis) as they release various inflammatory mediators (e.g., MPO) to help kill the invader, which may also damage the normal tissues (Abraham et al., 2000). Here, MPO is linked to chronic obstructive pulmonary disease.