[Note1: This entry was actually written in Sep 2009. I changed the posting date to keep my blog entries in chronological order.]
[Note2: There is an error in this blog post. RhD+ antigens only coat red blood cells, not kidney cells. Thus, RhD has no impact on matching donors to patients.]
My blood type is AB+. It’s somewhat uncommon (about 3.5% of the U.S. population has it), but what does it mean and what are its implications for donating a kidney?
All cells in our bodies are coated with proteins, lipids, and polysaccharides (long chain sugars). The molecular composition of this coating varies among individuals. One of the major sources of tissue incompatibility are polysaccharides that are grouped into what are called blood types O, A, B, and AB. People have two blood type genes, one from each parent. Those with two gene for type O have cells (including their red blood cells) coated with type O sugar. In addition, their body (including their blood plasma) carries antibodies that cause their immune system to attack cells coated with type A and type B as foreign.
People with two genes for type A, or the combination of one A and one O, have cells with the type A coating and their blood carries antibodies for type B sugar. Similarly, people with two genes for type B (or B & O) have cells with type B coating and antibodies for type A sugar. Finally, people with one A and one B gene, like me, have cells coated with both type A and type B sugars and do not carry antibodies for either type (which is good, otherwise I would die from an autoimmune reaction).
Another important antigen is called Rh factor, which is a protein. There are many types, but the one that causes the strongest immunological response is called D. It has several variations but they are simply grouped as RhD+ and RhD-. People who have the RhD+ versions never carry antibodies for the RhD- versions. People who have the RhD- versions can develop antibodies to the RhD+ versions, although it turns out most people don’t (as I will explain later).
Thus, when I donate my blood, the only person who can accept it must also be type AB, or else their immune system will react to it. However, recipients with type AB blood don’t carry any type A or B antibodies, so they don’t have to wait for a type AB donor. They can accept type blood cells coated with A, B, or O type polysaccharides. (Note: The transfusion can only contain blood cells, not plasma. Remember, the donors’ plasma may contains antibodies that would attach to the cells in the recipient’s body and cause an immune reaction.)
The same logic applies to organ donations. My kidney should only be donated to a person with type AB blood, but that patient doesn’t have to wait for my kidney. He/she can accept a kidney from a donor with any other ABO blood type (as long as all the A and B antibodies have been removed). Note that this means there will be a shortage of type O kidneys, even though type O is the most common blood type.
It also means there will be few patients with AB blood on the donor chain waiting lists maintained by the National Kidney Registry or the Alliance for Paired Donation. The patients on these lists all have unmatched donors. However, if a patient with AB type blood finds a person willing to donate to them, that person will likely be a match, even if they have a different blood type. So patients with AB blood type will take the organ offered by the donor with an unmatched blood type. They are unlikely to consider volunteering to be added to the donor chain waiting list. Joining a list would delay their surgery, complicate the logistics, and cause their donor to give the kidney to a stranger and the patient to receive a kidney from a stranger.
Incidentally, having type AB+ blood is the reason I’ve switched from donating blood to donating plasma. As stated above, my blood isn’t very valuable. However, since my blood doesn’t contain any antibodies for type A or type B sugars or RhD+ proteins, my plasma can be transfused into any patient (more on how plasma is used in a future post).
Where do the blood type antibodies come from? Humans who have RhD- blood type (i.e., don’t have cells coated with one of the variants of the RhD protein) are not born with the RhD+ antibodies and usually do not develop antibodies. However, if a mother is RhD- and becomes pregnant with a child who is RhD+, she may start producing RhD+ antibodies and thus develop a sensitivity to the antigen. This means she will be unable to accept blood from someone with RhD+ blood type (including her husband), if she ever needs a transfusion. It also puts any future fetus with RhD+ proteins at risk for hemolytic disease of the newborn.
In contrast, all humans who have O blood type (i.e., don’t have cells coated with the A or B sugars) have the A and B antibodies,people with A blood type carry B antibodies and people with B blood type carry A antibodies. The common explanation is that the antibodies are produced spontaneously by the immune system during fetal development. However, some evidence exists that this is not true. Instead, the antibodies are acquired after contact with the non-self antigens, either before birth or during the first year of infancy. Since the baby may have a different blood type than its mother, the source of the antigens is not the mother’s placenta or milk. The most likely explanation is an as-yet-unidentified viral or bacterial infection that introduces these antigens and thus stimulates the production of the antibodies. [I found this tidbit somewhere on the web, but can no longer find the citation.]
In order to match organs for transplant, the recipient cannot be sensitive to (that is, have antibodies for) the blood type antigens that coat the cells of the donor’s kidney. In addition, the recipient cannot be sensitive to the human leukocyte antigens (HLA). To learn more about HLA matching, see May 28.
[Update1: I’ve corrected an error regarding transplants to patients with AB blood type. It is common for them to receive organs from type O, A, or B donors. I previously said it was not common.]
[Update2: I’ve added an explanation of why I believe I haven’t found a match on the NKR or APD lists yet. A more detailed mathematical explanation is given in a Mar 2010 blog post.]