The rarest blood group in the world is Rh-null, sometimes called “golden blood.” Fewer than 50 people globally have ever been confirmed to have it. With a frequency of roughly 1 in 6 million, Rh-null is so scarce that finding a compatible donor in an emergency can require an international search spanning multiple countries.
But “rarest” depends on how you frame the question. Most people asking are thinking about the familiar A, B, AB, O system, where AB-negative is the least common type. Beyond that, there are 48 recognized blood group systems, and some produce phenotypes so unusual that entire registries exist just to track the handful of donors who carry them.
Rh-Null: The Rarest Known Blood Type
Red blood cells normally carry up to 61 different proteins in the Rh group alone. Rh-null cells carry none of them. As of the most recent published count, only about 43 individuals from 14 families worldwide have been identified with this phenotype. It’s inherited in an autosomal recessive pattern, meaning both parents must pass on the gene variant, which is why most confirmed cases trace back to families with a history of intermarriage between relatives.
The nickname “golden blood” comes from its universal compatibility within the Rh system. Because Rh-null red cells lack every Rh protein, they won’t trigger an immune reaction in any Rh-type recipient, making them extraordinarily valuable for transfusion. The flip side is that people with Rh-null blood can only safely receive Rh-null blood themselves. If you’re one of those 43 people, your donor pool is essentially the other 42.
Rh-null also comes with a mild but chronic form of anemia. Without the full set of Rh proteins, red blood cells are misshapen (a condition called stomatocytosis), which makes them more fragile and shorter-lived than normal cells.
The Bombay Phenotype: Rare and Easily Mistyped
Another contender for rarest blood type is the Bombay phenotype, designated Oh. People with this type lack the H antigen, which is the molecular foundation that A and B antigens are built on. Without H, you can’t express A or B, so standard blood typing reads these individuals as type O. They’re not. If they receive regular type O blood, their immune system attacks it because it still carries the H antigen they lack.
The Bombay phenotype occurs in about 1 in 10,000 people in India, where it was first discovered. In European populations, it drops to roughly 1 in a million. Like Rh-null, people with Bombay blood can only receive transfusions from other Bombay donors, and the risk of mistyping makes it particularly dangerous in emergencies. A person typed as “O” who is actually Oh could have a fatal transfusion reaction if the discrepancy isn’t caught.
Rarest Common Blood Type: AB-Negative
Within the everyday ABO and Rh system that hospitals use for routine transfusions, AB-negative is the least common type. Only about 1% of the U.S. population carries it. Here’s how rarity breaks down across the eight standard types, with some notable differences between ethnic groups.
Among white non-Hispanic Americans, about 17.3% are Rh-negative. That number drops sharply in other populations: 7.3% of Hispanic donors and 7.1% of Black non-Hispanic donors are Rh-negative, while only a small fraction of Asian donors carry the Rh-negative trait. This means types like O-negative and B-negative, already uncommon overall, are significantly harder to find in Asian donor pools. O-negative appears in about 8% of white non-Hispanic donors but only 0.7% of Asian donors.
These differences matter clinically. Patients from specific ethnic backgrounds sometimes need blood matched not just on ABO and Rh, but on additional antigen profiles that are more common within their own population. This is one reason blood banks actively seek diverse donors.
Beyond ABO: The 48 Blood Group Systems
The International Society of Blood Transfusion currently recognizes 48 distinct blood group systems. ABO and Rh get almost all the attention, but systems like Kell, Kidd, Duffy, Lutheran, and MNS carry antigens that can cause serious transfusion reactions if mismatched.
Some of these antigens are nearly universal, which means the rare individual who lacks them has very few compatible donors. In the Kell system, for instance, virtually 100% of donors in some populations carry the k antigen, making the rare K-positive, k-negative individual extremely difficult to match. In the Lutheran system, the Lu(a-b-) phenotype appears in fewer than 3% of donors in studied populations. Certain Duffy phenotypes are common in one group but vanishingly rare in another: about 49% of donors in one South Asian study lacked both Duffy antigens entirely, a pattern linked to natural resistance against a specific type of malaria.
When someone develops antibodies against multiple common antigens, perhaps after repeated transfusions or pregnancies, the pool of compatible blood shrinks with each new antibody. A patient who reacts to antigens in both the Kell and Kidd systems, for example, might need blood negative for several proteins simultaneously, a combination that could exist in less than 1% of the donor population.
How Rare Blood Gets to Patients
The American Rare Donor Program, a collaboration between the AABB and the American Red Cross, maintains a searchable registry called REGGI with phenotype data on more than 59,000 active rare donors across 82 member facilities in the U.S. and a few international sites. When a hospital identifies a patient needing rare blood, it submits a request to the Philadelphia-based program, which searches the registry and contacts member facilities to check unit availability.
Registered donors receive semiannual address-update cards so they can be recruited when their specific blood type is needed. The system works, but not perfectly. About 7.9% of requests for units lacking high-prevalence antigens go unfilled, meaning no compatible blood can be located. For patients needing combinations of common antigen-negative blood, the unfilled rate drops to 1.8%, and for certain antibody-deficient units it’s about 1.2%. When domestic searches fail, an international search begins, adding regulatory hurdles and shipping time to an already urgent situation.
Part of the challenge is that testing reagents for some rare antigens are themselves in short supply. Antisera used to screen for certain low-frequency and high-frequency antigens have become increasingly difficult to source, which makes identifying new rare donors harder over time.
Why Rarity Depends on Where You Are
A blood type that’s rare in one region can be relatively common in another. O-positive, the most common type overall, reaches 57% prevalence among Hispanic populations but is less dominant among white non-Hispanic groups. B-positive and AB-positive are more common among Asian donors than in other groups. Rh-negative types are disproportionately a European trait.
This geographic and ethnic variability means that “rare” is partly a matter of local supply. A patient with B-negative blood in Tokyo faces a much tougher search than the same patient in London. It also means that immigration patterns and demographic shifts in a region can change which blood types are hardest to stock. Blood banks in increasingly diverse cities need donor pools that reflect the populations they serve, not just the majority group.