HLA Genetics

Human leukocyte antigens (HLA) are proteins that are present on the surface of our bodies' cells. Our immune system recognizes our own cells as 'self,' as opposed to 'foreign,' based on the HLA proteins displayed on the surface of our cells. HLA typing is therefore very important in order to find a donor and recipient whose cells most closely match.

In bone marrow transplantation, HLA matching helps prevent rejection as well as graft-versus-host disease (GVHD). Bone marrow is not only the source of red blood cells that carry oxygen, but also the source of white blood cells that make up our immune system. In bone marrow transplantation, the donor's immune system in effect replaces the recipient's. If there is not a good match, this new immune system may recognize the recipient's body as 'foreign' and attack various organs and tissues.

Because HLA types are genetically determined, the best chance for a match is a donor who is genetically related to the recipient. An individual's HLA type is inherited from his or her parents at the time of conception. A cluster of genes on chromosome 6 determines HLA type. Given that one chromosome 6 is inherited from the mother and one from the father, each individual carries two copies of each of the genes in the HLA cluster. There are three major genes in this cluster that seem to be particularly important in transplantation: HLA-A, HLA-B, and HLA-DR. (Other genes can also be typed and may include HLA-DQ and HLA-C.) There are many different versions, called alleles, of each HLA gene. When doctors speak of a 6-out-of-6 match, they are referring to two people matched for the same alleles at each of their two HLA-A, HLA-B, and HLA-DR genes. <next>

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