natural killer cell

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A lymphocyte that is activated by double-stranded RNA or lymphokines and fights off viral infections and tumors without evident antigenic specificity.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun immunology Any cell that expresses cell surface receptors that deliver signals to either activate or inhibit a response as part of the immune system