MHC class I receptors on natural killer cells: on with the old and in with the new.

The functions of natural killer (NK) cells are controlled by an abundance of activating and inhibitory receptors. Many of these interact with MHC class I molecules. The MHC system also interacts with cytotoxic T-lymphocytes and has been shown to comprise a rapidly evolving family of molecules. This challenges the functional relationship of NK cell receptors with their ligands. Although individual receptors have become subject to species-specific expansions over evolutionary time, the main themes of the NK cell interaction with MHC class I have been preserved. This review details the interaction of NK cell receptors with MHC class I and discusses their unexpectedly rapid evolution.