Graft versus host disease (GVHD) occurs because the donated blood or marrow cells may attack the cells in the patient's body.
One of the functions of blood cells is to recognize infectious agents and other ‘foreign’ cells and attack them. This is called an ‘immune’ reaction, and is a very important way for the body to protect itself. In the human body, the T-cell, a type of white blood cell (WBC), is largely responsible for this action.
When the blood or marrow of the donor enters the patient during a transplant, it carries some its T-cells along with it. These immune cells circulate through the patient’s body in his or her blood. They come across the patient’s cells and recognize them as foreign cells. This may prompt them to launch an attack on the patient’s cells and cause damage to different organs.
How does the immune system recognize other cells as ‘self’ or ‘foreign’? It uses markers called HLA antigens that are present on the surface of cells to recognize which cells belong to one’s own body and which do not. (Read more about HLA and HLA-matching).
In an allogeneic transplant (stem cells donated by a different individual), the HLA of the donor and recipient are often not an exact match. Even when they are matching for all important HLA groups, small differences may remain in some other markers present on the cell surface. The donor’s T-cells can identify the mismatch and attack the recipient’s cells.
The closer the HLA match, the less likely it is that GVHD will develop. This is why a careful search for a closely matched donor is so important for a successful transplant.
