Reactive oxygen intermediates are released by:

Oxygen dependent killing is largely mediated through Reactive Oxygen Intermediates (ROI). The generation of reactive oxygen intermediates is due to the rapid activation of an oxidase, NADPH oxidase. Catalase, Superoxide dismutase and Glutathione peroxidase are radical scavenging enzymes that prevent oxygen mediated injury. Ref: Robbin's Illustrated Pathology, 7th Edition, Pages 16, 17, 60