Natural Killer Cells do not look inside cells. Even if they wanted to, they couldn’t—they have no way to look into the display windows, the MHC class I molecules, and read the story of the inside of the cell. No, instead they do something different: They check if a cell has MHC class I molecules. Nothing more, nothing less. This is solely to protect against one of the best anti–immune system tactics virus and cancer cells have. Generally cells that are either infected or unhealthy do not show MHC class I receptors in order to hide what is going on inside them. Many viruses force infected cells
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