Class I MHC i can serve as an inhibitory ligand for natural killer cells (NKs). Reduction in the normal levels of surface class I MHC, a mechanism employed by some viruses and certain tumors to evade CTL responses, activates NK cell killing.These inhibitory receptors recognize MHC class I molecules, which could explain why NK cells kill cells with low levels of MHC class I molecules. This inhibition is crucial to the role played by NK cells. MHC class I molecules are main way cells display viral or tumor antigens to cytotoxic T-cells. A common evolutionary adaptation to this seen in both intracellular microbes and tumours is a long-term down-regulation of these MHC I molecules. This makes the cells impervious to T-cell action. NK cells may have evolved as an evolutionary response: the loss of MHC would deprive these cells of the inhibitory effect of MHC and make them vulnerable to NK cell attack.