Why does the immune system regect organ transplants but not surgical implants e.g replacement knee?
The body can, and sometimes does reject artificial implants. For example, it is listed as a complication for knee replacement surgery. The reason why the organ transplant runs a higher risk is because the donor organ isn’t a neutral material. The “replacement” organ comes with it’s own immune system and DNA code that no matter how well matched, isn’t identical to the one it is replacing. And so they fight unless the immune system is suppressed.
