‘Graft versus host disease’ occurs when the transplanted lymphocytes (responsible for immunological reactions) from the donor function as previously programmed and recognize the recipient’s cells as enemy cells. The donor’s lymphocytes annihilate the recipient’s cells. The annihilation process affects virtually all of the recipient’s cell types. Short life cycle cells, such as epithelium and selected parenchymatous organs, are the cells most vulnerable to the attack. As a result, this condition is characterized by syndromes specific to epithelium damages (rashes, urticaria, dry patches of skin and epithelium desquamation e.g. of the digestive tract = diarrhoea). This condition is acute and resistant to treatment. The reactions are far less intense in case of cord blood transplants as the cord blood cells are immunologically “naïve”.