[rnet85]

It sounds harmless because it omits the incredibly complex process of developing immunity.

Your body basically is trying to do a sort of brute force/random combination of immune cells on the retrieved virus spike proteins until it comes up with the right combination.

https://en.wikipedia.org/wiki/Somatic_hypermutation

This process can result in B cells that produce antibodies which stick to virus proteins but also stick to your own cells. The B cells are essentially permanent, that’s how you develop lasting immunity, that unfortunately also means you have lasting immunity on one of your own cell types — autoimmunity https://en.wikipedia.org/wiki/Autoimmune_disease