<prompt> NK cells kill the viral infected cells due to:Increased expression of MHC class I moleculesDecreased expression of MHC class I moleculesIncreased expression of MHC class II moleculesDecreased expression of MHC class II molecules  <response>: Ans. a Increased expression of MHC class I molecules Features of target cell killing by NK cell: Inversely related to target cell expression of MHC class I molecule. So, it kills the cell that expresses little or no HLA class I molecule provide immunosurveillance such as virus infected cells, ceain tumor cells and allogenic cells. Non-immune, i.e. without previous sensitization, MHC unrestricted and non-antibody mediated. It kills host cell infected with intracellular bacteria, e.g. Mycobacteria, TB. Listeria, monocytogens. Do not kill cells which express class I MHC all normal nucleated cells express it. 