When Natural Killer Cells check out a civilian cell for infection or cancer, if they have plenty of MHC class I molecules, as most healthy cells do, the inhibitor receptor is stimulated and tells the Natural Killer Cell to chill. If the cell does not have enough MHC molecules though, there are no calming signals and the Natural Killer Cell, well, kills. Killing in this case means that it orders the infected cell to kill itself through apoptosis, the regular and orderly cell death, which traps viruses inside the carcass. So Natural Killer Cells are a bit like nervous agents who are walking
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