By ROBERT S. BOYD

WASHINGTON -- Millions of "natural killer cells" - nature's first line of defense against cancer, viruses and other infectious microbes - are on constant patrol inside your body.

These tiny assassins, the immune system's rapid-response team, can quickly spot a dangerous cell, poke holes in its outer wall and release poisons to destroy it. They also alert other immune cells to join the attack.

Despite their forbidding name, natural killer cells are the good guys in the never-ending war against disease.

Like some espionage agents, NK cells have a "license to kill," according to Wayne Yokoyama, an immunologist with the Howard Hughes Medical Institute at Washington University in St. Louis. The "license" is a molecular tag that allows a killer cell to whack a bad cell.

Although NK cells were discovered more than 30 years ago, they're still not well understood, and they remain an active field of research. Scientists are trying to figure out how killer cells "see" a target cell, how they tell whether it's infected or healthy and how they carry out their lethal task.


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