If a host cell expresses both MHC class I molecules and ligands that bind activating receptors on NK cells, what is the outcome for the host cell?

Question

Quizplus · Accepted Answer

The NK cell will recognize and bind to activating ligands and to MHC class I molecules on an infected host cell. However, the presence of MHC class I molecules serves an inhibitory function over the activating signals. The activating receptors are phosphorylated, and the inhibitory receptors recruit phosphatases to dephosphorylate the activating receptors, thus inactivating the activating receptors. This serves as a protective mechanism so that host cells are not inappropriately killed by NK cells. NK cells kill the infected host cell only when the MHC class I molecules are downregulated. Infected cells that present antigen on MHC class I molecules can be killed by cytotoxic T cells (CTLs).

If a Host Cell Expresses Both MHC Class I Molecules