Why can a body reject a transplant organ?

All your body cells carry a "label" or marker substance on the outside of the cell. This marker is recognized by the immune system, which will not attack it. In some very serious illnesses, when a person's organ has failed completely, an organ from another individual may be transplanted. The donated organ carry different marker substances, so the immune system treats them as invaders and will mount an attack on them called rejection. These attacks from the immune sysyem can be reduced or prevented by the use of powerful drugs.