system
Given the following post, respond as if you're commenting on Reddit:
human
Why are red blood cells safe from natural killer cells?

I'm currently reading Philipp Dettmer's excellent book, *Immune*. In it, he explains that unlike other immune cells, natural killer cells identify infected/corrupt cells by (among other things) detecting the absence of MHC Class I molecules on the surface of the cell, which are normally in abundance on all healthy nucleated cells in the human body.

In a footnote, he explains that infected red blood cells are safe(r?) from NK cells because they don't have MHC Class I molecules anyway.

What's not explained (as far as I can tell; I haven't quite finished the book) is how the NK cell knows the absence of MHC I on a red blood cell is normal. Is there some other marker that says "I'm a red blood cell" or is there more to activating NK cells than the reduction of this molecule? Or can the NK cells simply not trigger apoptosis in red blood cells for some reason? 

I'm not sure if I am missing something or my understanding is just fundamentally incorrect and I should go back and re-read a few chapters.
gpt
So the way that NK cells function is by responding to activating signals and inhibitory signals through their activating (NKG2C, KIR-2DS, 3DS)  and inhibiting receptors (NKG2A, KIR 2DL, 3DL). The inhibitory receptors recognize MHC I, which recruits protein phosphatases (SHP-1 and 2) and inhibit signaling. The activating receptor induces phosphorylation and activates the killing function of NK cells. 

Erythrocytes lack both activating and inhibitory signals, so the NK cells are not receiving a signal that tells them that the RBC is stressed and needs to be destroyed (no activating signal to tip the balance towards destruction). There are also many other signals that help the immune cells distinguish between self vs. non-self; the one that is usually referenced is CD47.