This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Immunity, Subject name: Pathology
First line of defence against tumor and virus? 
Select and explain. 
A. NK cell, B. T cell, C. Histiocyte, D. Macrophage
A: NK cell
Answer: A: NK cell 
Explanation: Ref: Robbins review E-Book, P 67* NK cells have the ability to respond without prior sensitization.* They carry receptors for MHC class I molecules which inhibit their lytic function. When expression of class I MHC molecules is reduced on a cell surface by viral interference, inhibitory receptors on NK cells do not receive a negative signal and the targeted cells is destroyed.* NK cells have CD16, Fc receptor that allows them to bind to opsonized cells and lyse them. This is a form of type II hypersensitivity with antibody mediated disease.