Why does the body attack a transplanted organ?

A transplanted organ is viewed by our bodies immune system as a foreign object, much like when we get sick. Our body is just trying to protect itself. For this reason people that have a transplant have to take immunosuppression medication for the rest of their lives.

A body attacks a transplanted organ because it does not recognize it as being made of self cells. It knows this because the new organ has antigens, a form of proteins that coat the organ. When a body's immune system recognizes these antigens and they are not similar enough to the body's own antigen's, it attacks to try and keep itself safe and healthy.