I learned my blood type the way most people do. A nurse drew a vial when I was a teenager, said “O positive,” and I filed it away as one of the few permanently settled facts about my own body. Four letters, a plus or a minus, done forever. Then I read about a handful of people whose red cells are missing a marker that more than 99.9 percent of the rest of us carry, and my tidy little fact came apart in my hands. Because for most of those people, the marker did not vanish at birth. It vanished later, and why it vanished turns out to matter.
The marker is called AnWj, and it has been a loose thread in hematology since 1972. Doctors could see it. They could test for it. They just could not say what it actually was, or why a vanishingly small number of people don’t have it. In September 2026 the International Society of Blood Transfusion formally logged the answer as its 47th recognized blood group system, ISBT 047, closing a case that a team at NHS Blood and Transplant and the University of Bristol first cracked open in a 2024 paper in Blood, the journal of the American Society of Hematology.
- 1972AnWj antigen first described
- 2024MAL gene identified as its source
- 2026recognized as blood system ISBT 047
The antigen was never the new part. It had been sitting in reference labs for fifty years. What the Bristol team found is the gene, and that is the harder and frankly better story. They pinned AnWj to the MAL gene and the small Mal protein it builds, and showed that the rare people who lack the antigen are missing full-length Mal protein from their red cells. Louise Tilley, the NHSBT scientist who led it, said she had personally been trying to resolve this one antigen for almost twenty years of her career.
Twenty years on a single protein. That is why nobody solved it in 1972. Mal is tiny, little was known about what it does on a red cell, and it never made anyone’s short list of suspects. The team had to run whole exome sequencing on the rare AnWj-negative individuals, then prove the link the hard way, by manipulating gene expression in developing blood cells to switch the antigen off and back on. Professor Ash Toye’s group could essentially direct young blood cells to stop making Mal and watch AnWj disappear along with it. When you can turn a trait off at the bench and see the marker vanish, you are no longer guessing.
And then comes the twist I did not see arriving: for most AnWj-negative people, the gene has nothing to do with it. The truly inherited form is almost unbelievably rare. Across the whole study the team confirmed only five people carrying the genetic version, caused by homozygous deletions in MAL, meaning both copies of the gene are gone, and one of them a member of an Arab-Israeli family. Those five are perfectly healthy; their bodies simply never build the protein. But they are the exception. In most cases, someone tests AnWj-negative because a haematological disorder or some types of cancer have suppressed the antigen, so it fades from cells that were making it just fine before.
Wait, why would a blood cancer reach in and switch off one specific surface marker? That is the question I could not let go of, because it flips what a negative test means. If almost nobody is born without AnWj, then a person who suddenly turns up negative is telling you something. The absence is not a quirk of ancestry. It can be a clue that something in the bone marrow, where blood cells are born, has started to change, one more thread a hematologist can pull on.
The stakes on the other side of this are just as physical. If an AnWj-negative person is exposed to normal AnWj-positive blood, their immune system can build anti-AnWj antibodies, and a later transfusion of ordinary blood can trigger a hemolytic reaction, where the transfused red cells are destroyed. Finding compatible blood for someone that rare is close to impossible when you are searching blind. Knowing the gene changes that. Genotyping tests can now be designed to identify these patients and the rare matching donors, and slotted onto the platforms blood services already run, so the search happens before an emergency instead of during one. Against more than 360 known blood antigens, that is one more that hospitals can finally screen for on purpose.
I keep coming back to how ordinary the machinery behind this was. No moonshot, no biotech valuation, no press tour. A publicly funded reference lab, partners including the Rabin Medical Centre in Israel, and one scientist willing to spend two decades on a protein most of her field had never thought about. That is the version of research I trust most, and it is the version that rarely gets the headline.
As for me and my “settled” O positive: I did one concrete thing after reading this. The only reason those five-in-a-study patients, and the sicker ones whose marker faded, ever find a match is that enough ordinary people bank enough ordinary blood to make a rare match findable at all. So I stopped treating my blood type as a closed file and signed up to donate. I would not have called that a health decision a week ago. Now I do.