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The immune attack by lymphocytes in the donor´s marrow or blood cell suspension (the graft) against the tissues of the recipient (the host). The immune cells most engaged in this reaction are donor T lymphocytes, which are present in the donor´s blood or marrow, the source of stem cells. The principal organs injured are the skin, the liver, and the gastrointestinal tract. The reaction does not occur in identical twin transplants. The reaction may be minimal in closely matched individuals or severe in less well-matched individuals. These reactions are mediated in part by antigens that are not in the HLA system and cannot be matched at this time prior to transplant. These are referred to as minor histocompatibilty antigens. For example, in the case of a female stem cell donor and a male recipient, the female donor´s cells that do not share the genes on the Y chromosome may see factors that are produced by genes on the Y chromosome as foreign. This fact does not prohibit female donors and male recipients, but it makes the risk of immune reaction higher.
A condition that occurs following bone marrow transplants in which the donor´s immune cells in the transplanted marrow make antibodies against the host´s (patient´s) tissues and attack vital organs. The condition may be acute or chronic, mild or severe. Severe cases can often be life-threatening.
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