Hamish is quite right - the human body will natruallly reject any 'foreign' interloper - this even extends to a developing foetus - the symptoms of rejection are consolidated in morning sickness, although in the interest of procreation, the female body does adapt to the interloper and accepts and develops it. Organ transpants remain as 'invaders' for the duration, and the host body has to be prevented from serious rejection symptoms, which can be fatal.