Neutrophils and other phagocytes manufacture O2 − (superoxide) by the one-electron reduction of oxygen at the expense of NADPH. Most of the O2 − reacts with itself to form H2O2 (hydrogen peroxide). From these agents a large number of highly reactive microbicidal oxidants are formed, including HOCl (hypochlorous acid), which is produced by the myeloperoxidase-catalyzed oxidation of Cl− by H2O2; OH· (hydroxyl radical), produced by the reduction of H2O2 by Fe++ or Cu+; ONOO− (peroxynitrite), formed by the reaction between O2 − and NO·; and many others. These reactive oxidants are manufactured for the purpose of killing invading microorganisms, but they also inflict damage on nearby tissues, and are thought to be of pathogenic significance in a large number of diseases. Included among these are emphysema, acute respiratory distress syndrome, atherosclerosis, reperfusion injury, malignancy and rheumatoid arthritis.