If a head transplant happened, would the head reject the body, or the body reject the head?

the body reject the head,
immune system is in the body - i think.

You shouldn't call it a head transplant, since the head is the part that you're trying to save from death. It's a body transplant. If there was not a good tissue match, then the body would reject the head. To my knowledge, none of the immune system components are produced in the head.