Oh man, that part of the immune system development is probably my favourite. Specifically how in the world the body us able to detect theoretically basically anything that can exist.

This is probably going to deep but you can read a bit on wikipedia if you are interested: https://en.wikipedia.org/wiki/V(D)J_recombination

And to pivot back what you asked, the part where the immune system can detect anything that exists would of course be bad if it detects your own body too, since it attacks what it can detect.

So theres like a training camp for immune cells where they are tested if they can detect your own body’s cells. And if they can, they are killed off. Therefore anything that is left can distinguish between what is good (you) and what is bad (other stuff).

There’s lots of other mechanisms around that though, otherwise allergies or intolerances wouldn’t exist, of course.