<prompt> Natural killer cells attacks which of the following cellsCells which express MHC1MHC cells which express MHC2Cells which are not able to express MHC1Cells which are not able to express MHC  <response>: The NK cells express activating and inhibitory receptors. The functional activity of the NK cells is regulated by a balance between signals from these receptors. Normal cells are not killed because inhibitory signals from normal MHC class I molecules override activating signals. The ability of NK cells to kill target cells is inversely related to target cell expression of MHC class I moleculesQ. If virus infection or neoplastic transformation disturbs or reduces the expression of class I MHC molecules, inhibitory signals delivered to NK cells are interrupted and lysis occurs. 