Blood Group Compatibility for Organ Donation

Choose the donor's blood group and the recipient's. The answer, the reason behind it, and what to do next appear immediately. Nothing you enter is sent anywhere — the check happens in your browser.

Every person carries antibodies against the blood group antigens they do not have themselves. Somebody with group A blood carries antibodies against B. Somebody with group B carries antibodies against A. Somebody with group O carries both, and somebody with group AB carries neither.

Those antibodies are present from early childhood, without ever having met the antigen, which is what makes blood group different from most immune barriers in transplantation. A recipient does not have to have been exposed to a donor to reject them on blood group. The antibodies are already there, waiting.

If an organ carrying group A antigen is transplanted into a recipient carrying anti-A, those antibodies bind to the blood vessels of the new organ within minutes. The result is hyperacute rejection: the graft is lost on the operating table, and the operation has made the recipient worse rather than better.

This is why blood type compatibility is checked before anything else, and why it is the one question a family can answer for themselves before they speak to anybody. Everything else in a transplant assessment takes weeks. This takes a moment.

This is the single most useful thing on this page, because it changes decisions.

The sign after a blood group — O positive, A negative — is the Rh factor, or RhD antigen. It matters enormously for blood transfusion, and it matters in pregnancy. Almost everyone has learned it in one of those two contexts, and almost everyone assumes it carries over.

It does not carry over to solid organ transplantation. The RhD antigen is expressed on red blood cells, not on the vascular endothelium of a kidney or a liver in a way that drives rejection. An O negative donor and an O positive recipient are compatible. So are the reverse, and every other combination of signs.

We write this out at length because of what happens when it is not said. A family finds a willing, healthy, genuinely compatible donor, notices that one is positive and the other negative, concludes the match is off, and stops looking. Nobody corrects them, because the conversation never gets as far as a transplant coordinator.

When you are checking compatibility, cover the sign with your thumb. Only the letter is doing any work.

An incompatible result is a redirection rather than an ending, and there are four directions.

Another donor in the family. Blood group is inherited, and siblings frequently differ from one another and from their parents. A family that stops at the first incompatible volunteer often has a compatible one sitting at the same table. Each potential donor should be checked separately rather than assumed from a relative's result.

ABO-incompatible transplantation. Some centres run programmes that transplant across the blood group barrier deliberately. The recipient is treated beforehand to remove the antibodies — plasma exchange, and medication that suppresses the cells producing them — and is monitored intensively afterwards. It is established practice in several countries and it works, but it is not a footnote: it adds cost, it adds weeks, and it adds risk that a compatible transplant would not carry. Ask a centre directly whether they run such a programme and how many they have done.

Paired exchange. Two incompatible pairs are matched so that each donor gives to the other recipient. Where a country runs an exchange programme at scale, this converts a great many incompatible pairs into compatible transplants without any desensitisation at all. Availability varies enormously by country, and a small programme may have nobody to match with.

The deceased-donor waiting list. In many countries this remains the main route to a transplant. Waiting times differ by blood group — group O recipients generally wait longest, because they can accept only group O organs while group O donors are in demand for everyone.

A compatible result answers one question and leaves several open. A family that treats it as a green light is often surprised later, so it is worth being precise about what has and has not been established.

The crossmatch. This is a laboratory test that mixes the recipient's serum with the donor's cells and looks for a reaction. It detects antibodies against that specific donor — antibodies a recipient may have developed from a previous transplant, a transfusion, or a pregnancy. A positive crossmatch is a contraindication even when the blood groups match perfectly, and it cannot be predicted from blood group at all.

Tissue typing. Human leukocyte antigen matching influences how likely the graft is to be rejected over years and how much immunosuppression will be needed. Unlike blood group and crossmatch, HLA is not usually a yes-or-no gate for living donation — a poorly matched living donor kidney commonly outperforms a well-matched deceased donor one — but it is part of the picture a transplant team weighs.

The donor's own health. A donor is assessed in their own right, by clinicians whose responsibility is the donor rather than the recipient. Kidney function, blood pressure, body mass, glucose tolerance, and the donor's own long-term risk all decide whether donation is safe for them. A perfectly compatible donor can be, and should be, declined if donating would not be safe.

The law where the transplant would happen. Whether a particular relationship may donate at all is a legal question, decided by the destination country, and it is independent of every medical fact above.