The formation of reactive oxygen species within inflammatory cells. Activation of the membrane bound NADPH-oxidase causes the reduction of the oxygen molecule to the superoxide anion, which dismutates, either spontaneously or via superoxide dismutase, to form hydrogen peroxide (1). Reactions with halide elements such as chlorine generate potent oxidants, for example hypochlorous acid (2). Alternatively, conversion to hydroxyl anion/radical can occur, catalyzed by iron in the Fenton reaction (3). Superoxide can also combine with nitric oxide to create the potent oxidant peroxynitrite (4). H2O2, hydrogen peroxide; HOCl, hypochlorous acid; MPO, myeloperoxidase; NADPH, nicotinamide adenine dinucleotide phosphate-oxidase; NO, nitric oxide; O2, oxygen; O2-, superoxide; OH-, hydroxyl anion; OH·, hydroxyl radical; ONOO-, peroxynitrite; SOD, superoxide dismutase.