NK cells target altered host cells, such as tumor cells, based on which signal?

NK cells target altered host cells, such as tumor cells, based on which signal?

• A. Altered MHC I molecules on target cell
• B. Altered CD4 molecule on target cell
• C. Interleukin 3 released by target cell
• D. Complement proteins attached to target cell

Answer: (A)

Natural killer cells rely on a missing self signal to recognize targets. When a cell downregulates MHC class I molecules on its surface, the inhibitory signals that normally keep NK cells quiet are lost. This absence of inhibition lets NK cells become activated and kill the altered cell, such as a tumor cell trying to evade T cells. The other options don’t provide the recognition cue NK cells use: CD4 is just a helper T-cell marker, not a signal NK cells use to identify targets; interleukin 3 is a cytokine affecting other cells rather than marking a target; and complement proteins on a cell reflect complement activation, not the NK cell's mechanism for detecting and killing stressed or transformed cells.

Natural killer cells rely on a missing self signal to recognize targets. When a cell downregulates MHC class I molecules on its surface, the inhibitory signals that normally keep NK cells quiet are lost. This absence of inhibition lets NK cells become activated and kill the altered cell, such as a tumor cell trying to evade T cells. The other options don’t provide the recognition cue NK cells use: CD4 is just a helper T-cell marker, not a signal NK cells use to identify targets; interleukin 3 is a cytokine affecting other cells rather than marking a target; and complement proteins on a cell reflect complement activation, not the NK cell's mechanism for detecting and killing stressed or transformed cells.