This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Medicine
The following feature is common to both cytotoxic T cells and NK cells: 
Select and explain. 
A. Synthesize antibody, B. Require antibodies to be present for action, C. Effective against virus infected cells, D. Recognize antigen in association with HLA class II markers
B: Require antibodies to be present for action
Answer: B: Require antibodies to be present for action 
Explanation: 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.