When Neutrophils engulf bacteria they exhibit a rapid increase in oxygen consumption known as respiratory burst. This phenomenon reflects the rapid utilization of oxygen and production from it of large amount of reactive derivatives, such as O2, H2O2, and OCl. It is called Respiratory Burst.
Respiratory burst From Wikipedia
To combat infections, immune cells use NADPH oxidase to reduce O2 to oxygen free radical and then H2O2. Neutrophils and monocytes utilize myeloperoxidase (MPO) to further combine H2O2 with Cl− to produce hypochlorite, which plays a role in destroying bacteria. Absence of NADPH oxidase will prevent the formation of reactive oxygen species and will result in chronic granulomatous disease.