Here’s what you’ll learn when you read this story:

• There are rare blood types beyond the eight most common ones, and researchers have discovered one more that brings the total count to 49.
• Blood type A(B) is caused by a four gene mutations and was discovered when the blood of patients and donors was being studied in a Thai hospital.
• Because similar mutations have been known to occur in other contexts, there could be more that are still undiscovered.

What’s your sign? What’s your aura? What’s your blood type? While horoscopes and New Age crystals often provide clear answers to the first two questions, the last one on that list can get complicated sometimes.

Blood types are identified by antigens—compounds made of sugars and proteins that alert the immune system to the presence of something foreign in the body—and they are the foundation of the ABO blood group system laid out by Karl Landsteiner in 1901.

The A antigen has N-acetylgalactosamine as its dominant sugar, the B antigen is dominated by the sugar D-galactose, and type AB blood has both antigens. Type O positive uses a different antigen altogether (known as the rhesus, or Rh, factor), and Type O negative has none of the above. All of this complexity is encoded into proteins by out ABO gene, located on our ninth chromosome.

While interesting in and of themselves, antigens become especially important when it comes to donating blood to and receiving blood from others. For blood transfusions to succeed, the antigens of the patient’s blood and the donor’s blood have to match, or else the new blood will be rejected by antibodies in the plasma. This is why type O negative blood is seen as the universal donor type—because it has neither of the A or B antigens nor the Rh(D) antigen, there is virtually no risk ofrejection.

But even rarer blood types than O negative exist. There are 50 people on the entire planet with Rh-negative blood, nicknamed “golden blood,” which is completely devoid of Rh antigens. And there is only one individual known to have Gwada negative blood. While O negative is only found in 7% of the global population, researchers who tested the blood of patients and donors in a Thai hospital discovered yet another new type in 2025 that runs through the veins of only three known individuals (it was found in one patient and two donors). What is now recognized as type B(A) has mostly B antigens, but also trace A antigens.

“ABO discrepancies were distinct between donors and patients even in the same ethnicity,” the researchers said in a 2025 study published in the journal Transfusion and Apheresis Science. “This finding highlighted the influence of the patient’s conditions and therapy on the anomalous ABO typing. Additionally, the B(A) individuals identified in this study carried identical genetic alterations that differed from all antecedent alleles of the B(A) phenotype.”

The researchers saw four alleles—alternative forms of certain genes at the same location on a chromosome—on the B(A) type that differed from alleles associated with other blood type and created B(A). Mutations like this had previously been seen in many ethnicities, including individuals from other parts of Asia, but it was the first time such a phenomenon was ever recorded in the Thai population. B(A) adds one more type to the 48 already known to occur in humans.

There could be more blood types out there that remain undiscovered, and more individuals with those extremely rare blood types. And the three currently known occurrences of B(A) may not be the only ones in the world. Further research may find more of either, and this discovery is proof that blood typing is not so simple as ABO.