Most of us know our blood type as a letter or two — A, B, AB or O, plus a positive or negative. But that familiar system is just the surface. Blood is far more complex, and scientists have now solved a puzzle that stumped them for more than 50 years, defining an entirely new human blood group.

Beyond ABO

Your red blood cells are studded with dozens of different molecules — antigens — and the immune system reacts to ones it doesn’t recognize. ABO and Rh are simply the most important for transfusion, but there are dozens of blood group systems in total (by late 2026, around 47–49 officially recognized). Each new one matters most for people whose blood is rare, because giving them mismatched blood can trigger a dangerous immune attack.

The mystery

The story centers on an antigen called AnWj, first identified in 1972. Scientists knew it existed — and that a few rare people lacked it — but for over five decades they couldn’t find the gene responsible. Without knowing the genetic basis, there was no reliable way to test for it or to systematically find compatible donors.

The breakthrough

Using whole exome sequencing — reading the protein-coding portions of the genome — researchers led by NHS Blood and Transplant in Bristol and the University of Bristol found the answer: people with inherited AnWj-negative blood carry deletions in the MAL gene, which makes a small membrane protein called Mal that carries the antigen. That established the new MAL blood group system (catalogued as ISBT 047). The findings were published in the journal Blood. “The genetic background of AnWj has been a mystery for more than 50 years… It represents a huge achievement to finally establish this new blood group system,” said senior research scientist Louise Tilley.

Why it matters clinically

This isn’t just taxonomy. More than 99.9% of people carry the AnWj antigen, which makes the tiny minority who lack it extraordinarily hard to match for transfusion. If an AnWj-negative person receives AnWj-positive blood, they can suffer a hemolytic reaction — their immune system destroying the transfused cells — which can be life-threatening. Now that the gene is known, doctors can use genetic testing to identify these rare patients in advance and to find compatible donors, heading off dangerous complications.

Why it matters more broadly

Every newly defined blood group system sharpens the safety net for transfusion medicine, especially for patients who need frequent transfusions — people with sickle cell disease, thalassemia or certain cancers — and who are most likely to develop reactions to mismatched blood over time. It’s a reminder that even in a field as old as blood typing, fundamental discoveries remain, and that modern genome sequencing is steadily filling in the gaps that classical methods couldn’t. This summarizes research and is not medical advice.