The production of various highly reactive oxygen derivatives (see reactive oxygen species) by certain cells, notably macrophages and neutrophils of the vertebrate immune system. These toxic compounds are used to kill bacteria engulfed by the macrophage, or secreted to attack larger parasites outside the macrophage. The enzyme NADPH oxidase catalyses the formation of superoxide anions (O₂⁻), which are then converted by superoxide dismutase to form hydrogen peroxide (H₂O₂), singlet oxygen, and hydroxyl radicals (OH·). Another phagocytic enzyme, myeloperoxidase, catalyses a reaction between chloride ions and hydrogen peroxide to yield hypochlorous acid (HOCl), which reacts with further hydrogen peroxide to yield more singlet oxygen. Nitric oxide, and its peroxynitrite derivatives, are also produced. The net effect of this toxic cocktail is to disable the target cell by oxidizing crucial components of its cellular apparatus.