[hfktk4nrn]

Cells have ways to detect non-self RNA and react to it, like stopping protein expression and releasing alarm chemicals to call in immune system cells. Viruses of course try to block these cell defenses.

This is actually a big problem for mRNA vaccines, which are also foreign RNAs, and to overcome it for example Pfizer/Moderna use unnatural RNA bases, among other things, to bypass these detection mechanisms.

It's not exactly clear how cells can distinguish self/non-self RNA, some markers are known, like different RNA cappings, different letter frequencies, some chemical modifications.