![](https://webarchive.library.unt.edu/web/20090115224318im_/http://i.usatoday.net/tech/_photos/2009/01/09/natural-killer-cellsx-rect.jpg)
By Mehrdad Matloubian and Karoline Hosiawa-Meagher, University of California San Francisco
By Dan Vergano, USA TODAY
In the rough-and-tumble world of your immune system, cells don't come any tougher than the "natural killer" cells the body deploys as...