MPO and chronic obstructive pulmonary disease: Moreover, neutrophils form reticular ultrastructures known as NETs and release various proteins outside of the cell by degranulation, including myeloperoxidase, neutrophil elastase, and Cit-H3.[33] Compared with healthy controls, accumulation of NETs is significantly increased in COPD sputum and serum, and high NET concentrations are positively correlated with the severity and exacerbation of COPD.[34] However, the role of NETs in COPD pathogenesis remains unclear.