Anti-oxidation generally requires the use of antioxidants, which refer to molecules that can slow down or prevent oxidation (often specifically in organisms). Oxidation is a chemical reaction in which electrons are transferred from substances to oxidants, and free radicals can be generated in the process, thus starting the chain reaction. When chain reaction occurs in cells, cells are destroyed or apoptosis. Antioxidants can remove free radicals, terminate chain reactions and inhibit other oxidation reactions, while being oxidized by themselves. Antioxidants are usually reducing agents, such as mercaptan, ascorbic acid and polyphenols.