Transplant rejection

Alternative names:

graft rejection; tissue/organ rejection

Definition:

A consequence of organ or tissue transplantation caused by an immune response that damages or destroys the transplanted organ/tissue. Also see graft-verses-host disease.

Causes, incidence, and risk factors:

The immune response protects the body from potentially harmful substances ("antigens") such as microorganisms, toxins, and cancer cells. The immune system distinguishes "self" from "foreign" and reacts against substances it recognizes as foreign. The presence of foreign blood or tissue in the body triggers an immune response that results in blood transfusion reactions and transplant rejection.

Blood and tissue contain identifying proteins on the surface that aid in distinguishing "self" from "foreign" tissues. These proteins can act as antigens that trigger the immune response, and antibodies are formed against foreign antigens. Tissue is "typed" according to the antigens it contains (Histocompatibility antigens).

No two people (except identical twins) have identical tissue antigens. Therefore, organ and tissue transplantation almost always causes an immune response against the foreign tissue (rejection), which results in destruction of the transplant. "Tissue typing" ensures that the organ or tissue is as similar as possible to the tissues of the receiving person. This is performed because greater antigen difference causes more rapid and severe rejection.

A few exceptions occur. Corneal transplants are rarely rejected because they have no blood supply, so lymphocytes and antibodies do not reach the cornea to cause rejection. Identical twins have identical tissue antigens, so transplantation between identical twins almost never causes rejection.