A transplant rejection is the body's destruction of a newly introduced organ or tissue. A person's immune system does not recognize the cells of the transplanted material, which triggers it into action to kill the transplanted tissues.
The immune system's response eventually makes the organ or tissue unable to function.
Medicines are given to suppress the immune system. But taking them can make it easy for a person to get sick. Having a suppressed immune system makes it hard for a person to fight off all types of infections. But transplants are usually successful.