<prompt> The following feature is common to both cytotoxic T cells and NK cells:Synthesize antibodyRequire antibodies to be present for actionEffective against virus infected cellsRecognize antigen in association with HLA class II markers  <response>: Answer is B Effective against virus infected cells : Common to both cytotoxic T cells and NK cells is affectivity against virus infected cells. Natural Killer cells: Are first line of defense against virus laden cells and neoplastic cells. Cytotoxicity is not MHC restricted. Cvtotoxic T cells: They are antigen specific T cells whose function is to produce antigen specific lysis of target cells by direct cell-tocell contact. Are usually CD class I MHC restricted T cells. Are impoant in defense against viral infections of tissues. Thus the feature common to both cytotoxic T cells and NK cells is affectivity against virus infected cells. 