While the surgery to donate a kidney is the same whether you donate to a relative or a stranger, there are different “types” of living kidney donation that allow people to donate when the donor and recipient aren’t a close “match”. See the information below to learn how we determine if two people match and about the different types of living kidney donation.
Compatibility is a measure of how closely two people “match” based on small markers (antigens) on the surface of their cells. Compatibility is important in organ transplantation because when a donor and recipient are “less compatible”, the recipient has a higher risk of rejecting their organ transplant. This means that their new organ won’t function as well and/or won’t function for as many years.
Compatibility is based on antigens and antibodies. Antigens are the small markers on the surface of our cells. Antibodies are small proteins produced by the body’s immune system. The immune system usually helps the body fight off infections, such as when you get a cold or you get an infected cut on your hand. However, the body’s immune system also tries to fight off anything that seems “foreign”. This includes an organ from another person, more commonly called an organ transplant. If a donor and a recipient have very similar antigens, it is harder for the immune system to notice that a “foreign” organ is now in the recipient’s body.
Types of Living Donor Kidney Transplants
Another way to think about compatibility is to think about noticing mistakes in a movie. If there are big, glaring mistakes, people in the audience are likely to notice. If there are small mistakes, a few people might notice the first time they see the movie, while other people might notice the small mistakes after seeing the movie a few more times. Similarly, if there are a lot of mistakes in a movie, you have more chances to notice one.
So, how does this relate to organ transplantation?
- The movie is everything going on in the human body. This movie is playing 24 hours a day, 7 days a week.
- The audience is the immune system, which is always watching.
- Big mistakes are like blood type incompatibilities. If a donor and a recipient’s blood types are different, the immune system is very likely to notice.
- Small mistakes are like HLA type incompatibilities. If a donor and a recipient’s HLA types are different, the immune system will notice after it has watched for a long time.
- More mistakes = a donor and recipient have more incompatibilities.
- Fewer mistakes = a donor and recipient have fewer incompatibilities (are a “close match”). In a way, the transplanted organ is “camouflaged” because it looks similar to the body around it.
When the immune system notices these incompatibilities, it produces antibodies to “attack” the foreign organ. It is trying to protect the person, but it is actually hurting their new organ. Therefore, we try and find donors and recipients who are compatible. As mentioned above, the closer the match, the lower the risk of rejection and the longer a transplanted organ will work. Below are more details about types of compatibility.
Blood type compatibility:
The first step in the living organ donation process is to test donors’ blood types, because blood type (A, B, AB, O) is the most important type of compatibility. A person’s blood type is determined by small markers (antigens) on the surface of red blood cells. Red blood cells circulate in blood vessels, delivering the oxygen that we breathe to cells in our bodies.
Blood type is inherited from our parents, so you are more likely to share your blood type with a close family member (e.g. mother, father, sister, brother, child) than with a stranger. The chart below shows which blood groups are compatible with one another. An “X” indicates compatibility.
|Recipient blood type|
|Donor blood type||O||X||X||X||X|
As you can see above, a donor of blood type O can donate to a recipient of any blood type. This is why type O donors are called “universal donors”. Similarly, recipients of blood type AB can receive an organ from a donor of any blood type. This is why type AB recipients are called “universal recipients”.
Human leukocyte antigen (HLA) type compatibility:
Another type of compatibility is based on human leukocyte antigen (HLA) type. HLA type is similar to blood type in some ways and different in others:
|Both HLA type and blood type are:||HLA type is different from blood type because:|
|– Inherited from your parents, so you are more likely to have the same HLA or blood type as a close relative than as a stranger.||– HLA antigens are on white blood cells (which helps us fight infections)|
|– Based on small markers (antigens) on the surface of cells||– Blood type antigens are on red blood cells (which deliver oxygen around the body)|
HLA compatibility is based on how many of the six markers (also known as antigens) on the donor’s and recipient’s cells match. Therefore, a donor and recipient can match on 0, 1, 2, 3, 4, 5, or 6 markers. The more markers that match, the more “compatible” the donor and recipient are and the less likely the recipient is to “reject” the donor’s organ.
Options for incompatible donors and recipients:
Paired kidney exchange:
This is a system where donor-recipient pairs look for other donor-recipient pairs. The first pair (Pair A) needs to find another donor-recipient pair (Pair B) where:
- The donor from Pair A is compatible with the recipient from Pair B.
- The donor from Pair B is compatible with the recipient from Pair A.
Then, the two pairs “swap” donors. This is why paired kidney exchanges are sometimes called “kidney swaps”. When a recipient swaps for a more compatible donor (a closer match), they lower their risk of organ rejection.
More than two donor-recipient pairs can also enter a swap together. Here is what it would look like if three donor-recipient pairs swapped kidneys:
How are donor-recipient pairs matched with each other?
Donor and recipient pairs are matched through a program by the Organ Procurement and Transplantation Network (OPTN), the organization that oversees all organ transplants in the United States.
How long does it take to find a matching donor-recipient pair?
The time a donor-recipient pair will wait for a match varies based on blood type, the levels of certain antibodies, and how many pairs are in the database at that time. As a result, it can take anywhere from one month to two years to find a “kidney swap” match for a donor-recipient pair.
How does a kidney exchange occur?
If both donors and both recipients can all come to the same transplant center, the two operations can happen at the same time or the same day. In some circumstances, especially if the donor-recipient pairs are at different transplant centers, the two transplants may be scheduled a few weeks apart. If more than two pairs of donors and recipients are involved, the entire “swap” may take longer.
What are the risks of a kidney exchange?
Donor-recipient pairs who enter a kidney exchange might have to wait longer for the transplant to happen than if they proceeded with a blood type-incompatible, sensitized, or positive crossmatch transplant (see below). An example of this would be if one of the recipients gets sick. Also, some donors don’t like the fact that they don’t know the person who receives their kidney.
Can pairs meet?
The donor-recipient pairs in a kidney exchange do not meet each other unless all participants agree that they want to meet. If the donor-recipient pairs want to meet, this will happen after the kidney exchange has occurred.
Blood type-incompatible transplants:
If a donor and recipient do not have compatible blood types and do not want to enter a paired kidney exchange, the donor might still be able to give a kidney to the recipient. This is called a “blood type-incompatible transplant” and is not performed at all centers. Recipients will have to undergo a process called plasmapheresis in which blood is taken out of the body, filtered, and put back into the body. If a recipient is on hemodialysis, the process for hemodialysis is similar. However, the part of the blood that is filtered out is different in plasmapheresis than in hemodialysis. In plasmapheresis, the machine filters out antibodies to the donor’s blood type, which are a made by the person’s immune system. By removing these antibodies, plasmapheresis lowers the risk that a recipient will reject an organ transplant from an incompatible donor.
What are the risks of a blood type-incompatible transplant?
On average, blood type-incompatible transplants do not last as long as compatible transplants, though it is impossible to predict the exact amount of time that any organ transplant will last. Since not all transplant centers will perform blood type-incompatible transplants, you might need to travel to a different center if you choose this option.
Sensitized and positive crossmatch transplants:
Even if a donor and recipient have the same blood type, the recipient may have antibodies that will attack the donor’s organ after transplant. When a recipient has these antibodies, the recipient is “sensitized”. A person can become sensitized in many ways, including pregnancies, prior organ transplants, or a history of blood transfusions. If a recipient is sensitized, they may wait 3-4 times as long for a deceased-donor kidney as patients that are not sensitized.
If a sensitized recipient has a potential living donor, the transplant center will mix a sample of the donor’s blood with a sample of the recipient’s blood. This test is called a “crossmatch”, and it shows if the recipient has antibodies against the donor’s antigens. A positive crossmatch means that the recipient has antibodies against the donor’s antigens, so the donor and recipient are incompatible. A negative crossmatch means the recipient does not have antibodies against the donor’s antigens, so the donor and recipient are compatible. (Of note: a new policy change that prioritizes kidney transplant candidates who are highly sensitized was passed in 2015 and is helping to make the waitlist time more equal between sensitized and non-sensitized candidates.)
Positive crossmatch ⇒ incompatible
Negative crossmatch ⇒ compatible
If a donor and recipient are incompatible, they can consider a paired kidney exchange or they can find a transplant center that does incompatible transplants. Recipients will have to undergo a process called plasmapheresis in which blood is taken out of the body, filtered, and put back into the body. If a recipient is on hemodialysis, the process for hemodialysis is similar. However, the part of the blood that is filtered out is different in plasmapheresis than in hemodialysis. In plasmapheresis, the machine filters out antibodies to the donor’s antigens, which are a made by the person’s immune system. By removing these antibodies, plasmapheresis lowers the risk that a recipient will reject an organ transplant from an incompatible donor.
What are the risks of a blood type-incompatible transplant?
Sensitized and positive crossmatch transplants do not last as long, on average, as compatible transplants, though it is impossible to predict the exact amount of time that any organ transplant will last. Since not all transplant centers will perform sensitized and positive crossmatch transplants, you might need to travel to a different center if you choose this option.