Why does the body attack a transplanted organ?
the organ has pathogens that the body wants to destroy.
the organ is not recognized by the body as being made of self cells.
the organ's ph level is not compatible with the recipient's ph level.
the organ does not have enough nutrients to support the body.

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.