What is the primary action of natural killer cells when they encounter cells expressing MHC class I proteins?

Natural killer (NK) cells play a crucial role in the immune response, particularly in the context of recognizing and eliminating infected or transformed cells. When NK cells encounter cells expressing MHC class I proteins, the primary action they take is inhibition of killing. This is because MHC class I proteins are typically expressed on healthy cells, providing a ‘self’ signal that shows the cell is normal and should not be targeted for destruction.

NK cells have specific receptors, known as inhibitory receptors, which engage with MHC class I molecules. The binding of these receptors sends a signal to the NK cells that inhibits their cytotoxic activity, preventing them from attacking and killing the cell. This is a vital mechanism that helps to maintain the balance between immune activation and tolerance, ensuring that NK cells do not harm healthy host tissues that express MHC class I.

In contrast, when cells lack MHC class I expression or express stress-induced ligands, the inhibitory signals are reduced, leading to the activation of NK cells and subsequent killing of those aberrant cells. Understanding these mechanisms highlights the importance of MHC class I in safeguarding healthy cells from unchecked NK cell activity.