Not only can the immunological reaction after organ or tissue transplantation result in the rejection of the transplant by the receiving organism (so-called host-versus-graft reaction); it may also happen – and is particularly frequent after bone marrow or stem cell transplantations – that the immune cells of the transplant from a donor recognise the recipient’s body as being foreign and attack it. This phenomenon is referred to as graft-versus-host reaction or disease, according to the ICD-10, the system for the classification of diseases, developed by the WHO. The graft-versus-host reaction is characterised by severe infections and changes in the mucous membranes of the gastrointestinal tract, the skin and the liver and may possibly be life-threatening. The severity of graft-versus-host disease can be attenuated to some extent by treating the transplant recipient with immunosuppressive drugs.