DefinitionTransplant rejection is when a transplant recipient's immune system attacks a transplanted organ or tissue. See also graft-versus-host disease.Alternative NamesGraft rejection; Tissue/organ rejectionCauses, incidence, and risk factorsYour body's immune system protects you from potentially harmful substances, such as microorganisms, toxins, and cancer cells. These harmful substances have proteins called antigens on their surfaces. If your immune system identifies antigens that are foreign (not part of your body), it will attack them.In the same way, foreign blood or tissue can trigger a blood transfusion reactionor transplant rejection. To help prevent this, tissue is "typed" before the transplant procedure to identify the antigens it contains.Though tissue typing ensures that the