Natural Killer Cells Target ________ Cells That ________

Question

Natural killer cells target ________ cells that ________.
A) infected; have lost MHC class I
B) bacterial; have lost MHC class I
C) bacterial; are coated with complement
D) infected; have lost C-reactive protein
E) infected; secrete cytokines

Accepted Answer

Natural killer cells recognize cells that have lost MHC class I molecules on their surface, which can be a sign of viral infection or cancer. The loss of MHC class I allows infected or cancerous cells to evade recognition by T cells, but makes them vulnerable to recognition and destruction by NK cells. Bacteria do not express MHC class I molecules, so option B is incorrect. Option C refers to opsonization, a process by which bacteria are marked for destruction by complement proteins and phagocytes, but this is not a direct target of NK cells. Option D refers to C-reactive protein, which is an acute phase protein involved in inflammation and not related to NK cell function. Option E is incorrect because NK cells do not target cells based on their secretion of cytokines.