Scientists Discover New Human Blood Type — B(A) Found in Only Three People

Researchers have identified a previously unknown human blood type, named B(A), after testing patients and blood donors at a hospital in Thailand. This discovery raises the number of documented human blood-group variants to 49 and underscores how much remains to be learned about blood-type diversity and transfusion compatibility.

How Blood Types Are Determined

Blood types are defined by antigens — molecules of sugars and proteins on the surface of red blood cells that can trigger immune responses. The familiar ABO system, established by Karl Landsteiner in 1901, distinguishes A and B antigens by their dominant sugars: N-acetylgalactosamine for A and D-galactose for B. People with AB blood express both antigens. The Rhesus (Rh) system is a separate antigen group; for example, O negative lacks A, B and Rh(D) antigens, which is why it is often called a universal donor type.

The New B(A) Type

In the Thai study, the B(A) phenotype was detected in three individuals (one patient and two donors). Laboratory and genetic analyses showed that B(A) blood displays predominantly B antigens with small amounts of A antigen. Sequencing at the ABO gene on chromosome 9 revealed four altered alleles at that locus that together produce the B(A) pattern.

“ABO discrepancies were distinct between donors and patients even in the same ethnicity,” the authors wrote in Transfusion and Apheresis Science, noting that a patient’s condition and treatment can influence anomalous ABO typing. They also reported that the B(A) individuals in their study shared identical genetic alterations distinct from previously described alleles.

Context And Rarity

Extremely rare blood profiles already documented include Rh-null (“golden blood”), found in about 50 people worldwide and lacking Rh antigens entirely, and Gwada negative, reported in a single individual earlier this year. By comparison, O negative occurs in roughly 7% of the global population and is commonly used in emergency transfusions because of its broad compatibility.

Why This Matters

The discovery of B(A) highlights two practical points: first, blood typing can be genetically and clinically complex, and second, more rare types likely remain undetected. As genetic sequencing and more detailed serologic testing become widespread, additional rare phenotypes and more carriers of known rare types may be identified — with direct implications for transfusion safety, donor registries and personalized medicine.

Next steps: Broader population screening, follow-up genetic studies, and awareness among transfusion services will help clarify how many people carry B(A) or related rare alleles and how best to manage transfusions for affected patients.