body fighting transplants (hey, that's close to being a contronym, how awesome is that)
Just to complete the set, there's also the inverse: the transplant fighting the host ("graft-versus-host"). Imagine how confused you'd feel as a sentient leukocyte, grafted onto a different body.
ヽ༼ຈل͜ຈ༽ﾉ Transplant hwaiting! ヽ༼ຈل͜ຈ༽ﾉ
In experiments performed on mice, blood transfusions from young mice reversed age-related markers in older mice. The protein involved is identical in humans.
http://mic.com/articles/88851/harvard-scientists-may-have-just-unlocked-the-secret-to-staying-young-forever