The Fenton reaction leads to free radical generation when

Reactive oxygen species(ROS) produced in small amounts in all cells during redox reactions that occur during mitochondrial respiration and energy generation.In this reaction, molecular oxygen is reduced by addition of four electrons to generate water.As this reaction is incomplete, toxic intermediates are produced.These intermediates include superoxide, which is conveed to H2O2 spontaneously by the action of enzyme superoxide dismutase.In the presence of metals such as Fe2+, H2O2 is conveed to highly reactive hydroxyl radical by Fenton reaction.Here the ferrous ions are reduced to ferric ions.MPO: myeloperoxidase Conves H2O2 to highly reactive compound hypochlorite.Present in leucocytes.{Reference; Robbins basic pathology 9E, page no.28}