HLA is also involved in other cellular immune mechanisms that affect transplant recipients. In graft-vs-host disease, donor derived immunocompetent lymphocytes react with HLA incompatible recipient cells and induce inflammatory responses in host tissues such as the skin and gastrointestinal tract. This complication is frequent after bone marrow transplantation, but may also affect recipients of liver and other transplanted organs and even blood transfusion. Graft-vs-host disease seems more likely in situations in which donor HLA is well matched to the patient, but not the other way around. For example, if an HLA-DR1,2 patient is exposed to immunocompetent cells from an HLA-DR1,1 homozygous donor, the patient's body will recognize the cells as a match, but the donor cells may identfy the patient's HLA-DR2 as a mismatch.
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