However, in this scenario, the body would have a distinct advantage (if you can call it that), given the disparity in mass and organ distribution. The body would have far more immune system capacity than the head in this case. White blood cells, for example, are produced in the bone marrow.

I'm no doctor, but given the importance of the brain and your spinal cord to even just the nervous system, I'd imagine that death would come rather quickly in an all out rejection.