Question

NK cells are immunologically significant as they recognize and kill:

• A. intracellular bacteria reservoirs
• B. cells with upregulated MHC-I molecules
• C. cells with downregulated MHC-I molecules
• D. cells with self-marker on their surface

Hint:

NK cells = "Natural Killers" of cells that try to hide by removing MHC-I.

Answer 1

The Correct Option is C

Step 1: Concept
Natural Killer (NK) cells are part of the innate immune system that monitor the health of host cells.

Step 2: Meaning
"Missing self" hypothesis states that NK cells look for the presence of MHC-I molecules, which are typically present on all healthy nucleated cells.

Step 3: Analysis
Many viruses and cancer cells downregulate (reduce) the expression of MHC-I on the cell surface to evade detection by T-cells. NK cells possess inhibitory receptors that bind to MHC-I; when MHC-I is absent or downregulated, the inhibitory signal is lost, and the NK cell is activated to kill the target cell.

Step 4: Conclusion
NK cells specifically target cells that have lost or reduced their MHC-I expression. Final Answer: (C)