Graft-versus-host disease (GVHD) is a condition in which the immune system of a person (the host) who has just received transplanted organs or tissues (the graft), attacks the organs or tissues which have been transplanted or grafted to the body. The response occurs when the host’s immune system recognises the grafted organs or tissues as foreign matter. In a similar manner to that in which the immune system fight against foreign bacteria or viruses, it mounts a response which causes the body to reject the new tissues or organs.

People who have a tissue or organ transplant are typically prescribed immunosuppressant medicines following a transplant to impair the immune system’s response and allow the new organ or tissue to establish itself.