" NK, or natural killer, cells, which, like macrophages, attack targets like microbes, do not always kill. A 2013 article reports that about half of the NK cells sit out the fight, leaving a minority of them to become what their human observers call serial killers. "

― Barbara Ehrenreich , Natural Causes: An Epidemic of Wellness, the Certainty of Dying, and Killing Ourselves to Live Longer