Antioxidants are substances that slow down or prevent the oxidation of other substances. Oxidation is a chemical reaction that transfers electrons from a substance to an oxidant. Oxidation reactions can produce free radicals which start a chain reaction that damages the cells; antioxidants terminate these chain reactions by acting on the intermediate radicals and inhibiting other oxidation reactions by oxidizing themselves. As a result, antioxidants are defined chemically reducing agents such as thiols or polyphenols - as chemical reactions are involved in oxidation-reduction.
Although oxidation reactions are crucial for life, they can be just as damaging; hence, plants and animals maintain complex systems of multiple types of antioxidants, such as glutathione, vitamin C and vitamin E, as well as enzymes such as catalase, superoxide dismutase and various peroxidase. Levels that are too low of antioxidants or inhibition of the antioxidant enzymes, cause oxidative stress and may damage or kill the cells.