The Science of Organ Transplantation – Part III
Basics of Tissue Matching
All of us have an immune system, the purpose of which is to defend the body from invasion of foreign organisms and other insults that can harm the body. This defense mechanism gives us the resistance to fight the many infections from bacteria, virus, fungus, etc., and is called immune response. Unfortunately, the same immune response can attack and destroy a foreign tissue implanted in the body like a transplanted kidney. This is called “rejection reaction” and is one of the major concerns in the field of organ transplantation. Clearly our body was not designed with modern treatments like organ transplantation in mind. The only way to counter this is to either find the best matched living donor like a close relative or look for ways to suppress the immune system to prevent or control the rejection reaction. In order to find the best match, you will need “Tissue typing,” a procedure in which the tissues of a prospective donor – practically only blood is used – and recipient are tested for compatibility prior to transplantation.
When Joseph Murray did the first kidney transplant in 1955, he had to select an identical twin who would obviously be the most compatible person and hence likely to be the best match. It certainly paid off and he could do the surgery without tissue typing and compatibility testing which was non-existent at that time. Most early living donors were close biological relatives of the recipients since the risk of rejection is likely to be less. However, it’s not always so with other donors — cadaveric, living unrelated or even related donor as well. You will need tissue typing and histocompatibility testing and only if the donor organ is a reasonably good match, the transplantation will proceed.
This brings us to the question of “What exactly are “histocompatibility matching, tissue typing and HLA Testing?” This is a bit more complicated to explain than the simple matching of blood groups. Basically, we are trying to match the right donor to the recipient. This is done with a blood sample drawn from both parties to analyze the special antigens in their lymphocytes called Human Leukocyte Antigens (HLA) and see if they are similar. These are called ‘HLA Markers’ and we inherit half of what we have from our mother and the other half from our father, so each brother and sister who shares the same parents has a 25 percent chance (1 in 4) of being a close HLA match. Extended family members may or may not be close HLA matches. Also, about 70 percent (7 out of 10) of patients who need a transplant won’t have a fully matched donor in their family.
Research has found that a “donor must match a minimum of 6 HLA markers and many times a closer match is required.” Only through detailed testing, a best match is found. Because some HLA types are more common than others, some patients may face a greater challenge in finding a matching donor. If you are of Indian origin living in the U.S. and trying to get a cadaveric kidney, a close match is likely to come from a donor of similar heritage. Fortunately, UNOS (United Network of Organ Sharing) computer system that manages the national organ transplant system is very efficient and makes it all possible to get the organ through its wide reach. The way it works is, when a patient is “added to the list,” the transplant hospital feeds that patient’s medical information into UNOS’ computer system. When a deceased organ donor becomes available, UNOS’ computer system generates a ranked list of transplant candidates, or “matches”, based on blood type, tissue type, medical urgency, waiting time, expected benefit, geography and other medical criteria.”
As I mentioned before, a close match between a donor’s and the patient’s HLA markers is usually needed for a successful transplant outcome, especially if it’s a cadaveric donor. However, with modern immunesuppressant drugs, even for those with lesser matching, the patient can expect satisfactory long term results. When I asked a transplant specialist about how close the matching has to be for adequate results, he said, “As long as your blood group matches with the living related donor, we will still proceed with the transplant. That’s the minimum criteria.” That’s great news for the patients. ‘Living related’ is the key factor here. This is the main reason we encourage close relatives of the patients to come forward and participate in the organ donation program.
To be continued …
Recommended for further reading: “Second Chance – A Sister’s Act of Love” by M. P. Ravindra Nathan MD, Outskirts press. www.amazon.com
M.P. Ravindra Nathan, M.D., is a cardiologist and Emeritus Editor of AAPI Journal.