When the human body receives an organ or tissue from another person, the immune system recognizes that organ or tissue as being foreign and tries to “reject” the organ. Fortunately today, doctors have powerful medications that prevent and treat rejection.
How does the body recognize a donated organ (such as a donated kidney) as being foreign?
The answer lies in the types of proteins and carbohydrate molecules on the surfaces of the tiny cells that make up the organ. There are two types of important cell surface molecules that are responsible for rejection in humans: 1) Carbohydrate Molecules that determine blood types (either O, A, B, or AB) and 2) Protein Molecules called HLA antigens. Before a transplant, tests are performed to make sure that the recipient does not have immunity to the donors ABO blood type molecules and the donor’s HLA antigens.
In a first step, doctors make sure that the donor and recipient have blood types that are compatible. If the blood types are not compatible, immediate rejection of the kidney is likely to occur. Most transplant centers around the world will not perform a kidney transplant if the blood types are not compatible.
In a second step, doctors perform a test called a cross match test. This determines whether the recipient has immunity to the donor’s HLA antigens. So how does a person develop immunity to someone else’s HLA antigens? The answer is simple: immunity to HLA antigens may develop when a person’s immune system comes in contact with another person’s cells, tissues or organs. This can occur with blood transfusions, organ or tissue transplants and even pregnancy. Pregnancy is the most common cause of HLA immunity, because the baby has the HLA antigens of the father, which often come in contact with the mother’s immune system.
What happens if HLA immunity is present, and a transplant is performed?
There is a high risk of the recipient experiencing a very rapid rejection (within hours of the transplant), called Hyper Acute Rejection.
If the donor and recipient have compatible blood types and the cross match test is negative, (that is the recipient does not have immunity to the donor’s HLA antigens) the transplant can be safely performed.
However, one of the most common reasons that a willing living donor cannot donate to their loved one is because they have the wrong blood type or there is a positive cross match with the recipient. These donors and recipients may benefit from paired donation, which will allow the donor to donate and the recipient to receive a living donor kidney.