Free radicals are molecules which have an unpaired electron although it is also possible to have atomic free radicals. They are thus highly and non-specifically reactive, binding to a variety of cellular components in vivo and thus causing damage to, for example DNA. Free radicals are produced continually in nature, but they are very short-lived owing to their reactivity. This is enhanced in biological systems by free radical scavengers which are often deliberately produced in nature. Within the human body, they are produced in a concentrated form by the immune system.