AB negative is the rarest of the eight standard blood types, found in roughly 1% of donors. But beyond the familiar ABO system, there are blood types so uncommon that fewer than 50 people on Earth are known to have them. The answer to “what are the rare blood types” depends on how deep you go: the standard eight types have a clear rarity ranking, while dozens of lesser-known blood group systems contain phenotypes that can make finding a compatible transfusion nearly impossible.
The Rarest of the Eight Common Types
Most people know blood is typed by two systems: ABO (A, B, AB, or O) and Rh factor (positive or negative). Combining these gives eight types. Based on NHS Blood and Transplant data from donor populations, the distribution breaks down like this:
- O positive: 36%
- A positive: 28%
- O negative: 14%
- A negative: 8%
- B positive: 8%
- B negative: 3%
- AB positive: 2%
- AB negative: 1%
AB negative sits at the bottom because you need two relatively uncommon traits at once: the AB antigen combination (already the least common ABO group) and a negative Rh factor. Still, 1% of the donor population means it’s findable in any large blood bank. The truly rare types exist outside this eight-type framework entirely.
Rh-Null: The “Golden Blood”
Rh-null is sometimes called the rarest blood type in the world, and for good reason. People with Rh-null blood are missing all of the antigens in the Rh system, not just the D antigen that determines whether you’re “positive” or “negative.” The Rh system actually contains over 50 different antigens, and Rh-null individuals express none of them.
It occurs in roughly 1 in 6 million people and is inherited in an autosomal recessive pattern, meaning both parents must carry the gene. As of the most recent published count, only about 43 people from 14 families worldwide have been identified with this phenotype. The trait tends to appear more often in families with a history of consanguinity (relatives marrying relatives), which increases the odds of both parents carrying the same recessive gene.
The nickname “golden blood” comes from its universal compatibility within the Rh system. Because Rh-null red blood cells carry no Rh antigens, they won’t trigger an immune reaction in anyone who has antibodies against Rh proteins. That makes it extraordinarily valuable for transfusion. The flip side is that people with Rh-null blood can only safely receive Rh-null blood themselves, and with fewer than 50 known carriers, finding a donor is a serious medical challenge. Many Rh-null individuals bank their own blood in advance for potential future surgeries.
Rh-null also comes with a mild form of chronic anemia. Without Rh proteins on the surface of red blood cells, the cells are more fragile and break down faster than normal. This is usually manageable and doesn’t cause severe symptoms, but it means the blood type is not just a labeling quirk. It has real physiological consequences.
The Bombay Phenotype
The Bombay phenotype is another exceptionally rare blood type, and it creates a problem that surprises even experienced lab technicians. People with this type appear to be type O on standard testing, but they are not. They are missing the H antigen, a precursor molecule that the A and B antigens are built on top of. Without the H antigen, the body cannot produce A, B, or even a normal O surface. The result is red blood cells that are truly “blank” in the ABO system.
This matters because Bombay phenotype individuals will react against transfused blood from any standard ABO type, including type O. Their immune system recognizes the H antigen (present on all normal blood, including type O) as foreign and attacks it. They can only receive blood from other Bombay phenotype donors.
Prevalence varies dramatically by region. In India, where it was first identified in 1952, it occurs in about 1 in 10,000 people. In Taiwan, the rate is roughly 1 in 8,000. In European populations, it drops to about 1 in a million. The cause is a mutation in the FUT1 gene that introduces a premature stop signal, producing a truncated, nonfunctional enzyme. Because the inheritance is recessive, both copies of FUT1 must be inactive for the Bombay phenotype to appear.
Rare Types Within Minor Blood Group Systems
ABO and Rh get all the attention, but as of June 2024, the International Society of Blood Transfusion recognizes 47 distinct blood group systems encompassing 366 antigens. Many of these systems contain phenotypes rare enough to cause serious transfusion problems.
The Kell system is one of the most clinically significant after ABO and Rh. The K+k- phenotype, where a person carries only the K antigen and lacks the k antigen (which over 99% of people carry), is extremely uncommon. In one study of South Gujarat donors, not a single K+k- individual was found. Because the k antigen is so widespread, people who lack it and develop anti-k antibodies face major difficulty finding compatible blood.
The Kidd system includes the Jk(a-b-) phenotype, in which a person lacks both Kidd antigens. This is vanishingly rare in most populations, though it appears somewhat more frequently in Polynesian and East Asian groups. The Duffy system similarly has a null phenotype, Fy(a-b-), which is actually very common in people of African descent (where it provides some resistance to a particular malaria parasite) but rare in other populations. This creates a mismatch: blood that is Duffy-null is easy to find in parts of sub-Saharan Africa but scarce in European or Asian blood banks.
Why Ethnicity Affects Blood Rarity
A blood type’s rarity is always relative to the population around you. Certain rare antigens and phenotypes cluster in specific ethnic groups because of genetic drift, founder effects, or (as with the Duffy-null example) evolutionary pressure from infectious disease. The practical consequence is that someone from a minority ethnic group living in a country where most donors don’t share their background may find it exceptionally hard to get matched blood.
Some rare subtypes, like U-negative blood, are found almost exclusively in people of African ancestry. The same is true for certain Rh variants. NHS Blood and Transplant explicitly notes that some rare blood types are only found within Black, Asian, and minority ethnic communities, which is why blood services in diverse countries actively recruit donors from these populations. A larger and more diverse donor pool directly increases the chances of matching a patient with an unusual phenotype.
What Happens When No Match Exists
For patients with ultra-rare blood types, the consequences go beyond inconvenience. When someone’s blood reacts against all available donor red blood cells during pre-transfusion testing, typically because they carry antibodies against a high-frequency antigen that nearly everyone else has, the clinical team faces a situation with no clean solution. A published case report described a pregnant patient whose blood reacted with every unit tested, leaving no compatible products available without significant risk of hemolysis, the destruction of transfused red blood cells by the patient’s immune system.
Hemolytic transfusion reactions range from mild fever and chills to organ failure and death. In pregnancy, incompatible blood poses additional danger because certain antigens are expressed more strongly on fetal and newborn blood cells, raising the risk of hemolytic disease in the baby.
To prevent these scenarios, rare donor registries exist worldwide. The American Rare Donor Program (ARDP) classifies a donor as “rare” if they are negative for a high-prevalence antigen (one that fewer than 1 in 1,000 people lack), negative for multiple common antigens across several blood group systems, or IgA-deficient. Each rare donor is extensively typed across the major antigen systems, and their information is entered into a searchable database. When a patient needs rare blood, registry staff can identify which facilities have matching donors on file and coordinate recruitment. It’s a slow, deliberate process, which is why elective surgeries for patients with rare blood are planned months in advance whenever possible, and autologous donation (banking your own blood before a procedure) is strongly encouraged.