Sometimes, your body has to kill things. Most of these things are bacteria, viruses, and other invaders, but sometimes it also has to take out your own cells. As they divide, cells accumulate mutations and can start to act funny. Ideally, before they turn into something serious like cancer (which is just cell growth gone haywire), your body kills them. The ways in which your immune system attacks both its own cells and foreign microbes are hugely varied, but free radicals are one weapon in the arsenal. Your immune system is essentially harnessing their destructive power for good. When you get a cut on your finger, one crucial step your body takes to prevent infection is to produce more free radicals in the area. Almost all inflammation in your body involves free radical production, which is one reason that chronic inflammation can lead to a variety of diseases, especially cancer. Over time, that exposure to DNA-damaging molecules results in mutations, and those mutations can accumulate to make a cell cancerous. But in small doses, free radicals are crucial.