Attention has recently focused on the possible significance of natural killer (NK)2 lymphocytes in the surveillance of tumors by the immune system (1). This is because NK cells from several species preferentially lyse transformed target cells in vitro (2–5). NK lymphocytes from mouse and man have both been studied in detail, but the manipulability of the mouse model has allowed for the better resolution of NK cell properties. In this review I will restrict most of my comments to the mouse NK cell system. The properties of human NK cells have recently been reviewed elsewhere (6).

Characterization and tissue distribution. It is agreed that mouse NK cells are x-ray-resistant (7) lymphocytes lacking cell surface immunoglobulin (8, 9). They are thought to be derived from bone marrow since their activity is removed by depletion of bone marrow cells with anti-bone marrow antibody or with 89Strontium (7, 10).

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