In chemistry, radicals (often referred to as free radicals) are atoms, molecules, or ions with unpaired electrons on an open shell configuration. The unpaired electrons cause them to be highly chemically reactive. Radicals play an important role in combustion, atmospheric chemistry, polymerization, plasma chemistry, biochemistry, and many other chemical processes, including human physiology. For example, superoxide and nitric oxide regulate many biological processes, such as controlling vascular tone. “Radical” and “free radical” are frequently used interchangeably, although a radical may be trapped within a solvent cage or be otherwise bound.
Free radicals play an important role in a number of biological processes, some of which are necessary for life, such as the intracellular killing of bacteria by phagocytic cells such as granulocytes and macrophages. Free radicals have also been implicated in certain cell signalling processes. This is dubbed redox signaling.
Because free radicals are necessary for life, the body has a number of mechanisms to minimize free radical induced damage and to repair damage that occurs, such as the enzymes superoxide dismutase, catalase, glutathione peroxidase and glutathione reductase. In addition, antioxidants play a key role in these defense mechanisms. These are often the three vitamins, vitamin A, vitamin C and vitamin E and polyphenol antioxidants.