I thought it was that certain roaming immune cells are basically pretty interested in literally everything they encounter, but you’re own cells have chemicals cues that signal they are ok. And as soon as a they doesn’t get the ok signal, all hell breaks loose, signaling the home bade, calling for backup, etc.

But this can go wrong, with like bacteria or cancer trying to mimic the “ok” Signal.

If someone wants to add, please do, because although i’m a biologist i mostly remember that the immune system is way more complex than you’d think.