Recognition events that inhibit and activate natural killer cells.

Natural killer (NK) cells exhibit specificity for MHC class I molecules in their interactions with target cells. Instead of activating the cells, the recognition of class I molecules on target cells inhibits NK-cell lytic activity, suggesting that surveillance of normal class I expression by body cells represents a key NK-cell function. Recent advances have been made in identification of the class I specific receptors expressed by NK cells, and in the characterization of their detailed specificity and expression patterns. Other studies are beginning to unravel recognition events that activate NK cells.