'Supernatural' killer cells.

The observation that lymphocytes from healthy individuals, without any known sensitization, could spontaneously lyse tumour target cells in vitro(1,2) was first thought to be an in-vitro artefact. Now the effector cells, known as natural killer (NK) cells, are accepted as a clearly distinct albeit heterogeneous subpopulation of lymphocytes. Yet their lineage remains controversial and despite much interest in the role of NK cells in protection against infection and cancer, their biological significance is far fiom clear(3-5). In this article Gordon Burns and his colleagues discuss recent research on NK cells which has illuminated the diverse effects of lymphokines, expanded knowledge on the mechanisms of cell recognition and killing by cytotoxic effector cells, and illustrated how a variety of leukocytes mediate more than one function - results of general interest to cell biologists.