Most of us only think about our blood type when we tick a box on a form or glance at a blood donor card. Yet a new study from Mahidol University in Thailand shows that behind those simple letters lies a much more intricate story. After checking 544,230 blood samples from patients and donors, researchers found just three people with a hybrid B(A) blood type, a quirk estimated at about 0.00055% of the population, roughly one in 180,000.
That tiny fraction may sound like a medical curiosity. For transfusion teams, it is a practical warning that rare blood variants can quietly slip past routine tests and complicate lifesaving care.
A microscopic twist in the ABO system
Human blood is usually grouped into eight main types based on two pieces of information. The first is the ABO system, which depends on sugar-based antigens A and B that sit on the surface of red blood cells. Type O carries neither A nor B. The second is the Rh factor, a protein that adds the familiar plus or minus to your type.
Each blood type acts like a molecular ID card. Your immune system learns to recognize your own markers and may attack blood that looks foreign. That is why a mismatched transfusion can trigger a dangerous reaction instead of helping the patient.
In the B(A) phenotype uncovered in this study, the blood behaves mostly like type B, yet the enzyme that decorates red blood cells with sugars has picked up a tiny hint of A like activity.
On lab tests, red cells show a weak A signal while the plasma still looks like it belongs to a person with type B. The result is an ABO discrepancy, a situation where standard tests return conflicting answers and force staff to pause before hanging a bag of blood.
What the Thai researchers actually found
To see how often these puzzles occur, the team reviewed more than 285,000 donor samples and more than 258,000 patient samples collected over eight years at Siriraj Hospital in Bangkok. ABO discrepancies appeared in 396 patients, about 0.15%, and 74 donors, about 0.03%.
Within that small pool, only one patient and two donors showed the B(A) phenotype. When scientists sequenced their DNA, all three shared the same four tiny changes in the ABO gene, creating a version of the enzyme that is new to science.
Although the variant seems ultra rare, the team notes that such findings hint at a much wider spectrum of blood diversity that routine typing simply does not see. As they put it, “Future studies are required to elucidate the structural and functional consequences of the mutated AB transferase.”
Why a blood type almost no one has still matters
For someone reading their lab report at home, this discovery does not mean your blood type is suddenly unsafe. It does highlight how much transfusion safety depends on careful cross checking of both red cell antigens and plasma antibodies, and in some tricky cases on genetic testing when numbers do not quite add up.
In practical terms, rare phenotypes like B(A) push blood banks to refine their algorithms, upgrade analyzers, and train staff to recognize discrepancies before an urgent transfusion, whether that is after a car crash or during routine surgery. They also remind researchers that human genetic diversity is still being mapped, one quiet anomaly at a time.
Could your own blood carry a subtle twist that standard tests overlook? For most people that question will never change day-to-day care, yet work like this improves the safety net underneath everyone who might one day need donated blood.
The study was published in Transfusion and Apheresis Science.
