[Free radicals in clinical biology. Origin, pathogenic effect and defense mechanisms].

Free radicals are chemical species with an unpaired electron on their outer orbit. This review discusses the various endogenous and exogenous sources of free radicals; toxicity of free radicals, especially for DNA and cell membranes; a number of pathophysiologic states related to overproduction of free radicals; the natural protective systems--involving enzymes or scavengers--which prevent free radical-induced cell injury, and therapeutic trials of antioxidants for improving protection against free radicals in a number of conditions.