Your blood type—whether A, B, AB, or O, positive or negative—is determined by specific proteins on the surface of your red blood cells. This article explains what defines a blood type, which groups are rarest, how they are inherited, and what compatibility rules matter for transfusions and pregnancy, drawing directly from official blood donation organizations and medical institutions.
Last checked: 2026-07-02
Blood groups: A, B, AB, O (each positive or negative) ·
Rarest blood group: AB negative ·
Number of blood groups: 8 ·
Classification systems: ABO and Rh
How we researched this
Last checked: 2026-07-02.
Sources reviewed: Official blood donation organization publications, government health authority websites, medical research institution articles, hospital transfusion medicine resources.
We conducted no on-site laboratory analysis, no medical advice was given, no individual testing was performed, and no primary research was undertaken.
Blood types at a glance
What defines a blood type
- Blood type is determined by the presence or absence of specific antigens (A and B) on red blood cells, plus the Rh protein, creating eight common types. (American Red Cross)
Rarest common type
- Among the eight standard ABO/Rh types, AB negative (AB−) is the least common, occurring in about 0.4% of the U.S. population. (Healthgrades)
Most common type
- In the UK donor registry, O positive is the most common blood type (about 36% of donors), followed by A positive (28%). (NHS Blood Donation)
Universal donor and recipient
- O negative is the universal donor for red blood cells because it lacks A, B, and RhD antigens. AB positive is the universal recipient, able to accept all ABO/Rh types. (El Camino Health)
| Feature | Description |
|---|---|
| Blood groups | A, B, AB, O (each positive or negative) |
| Rarest blood group | AB negative |
| Number of blood groups | 8 |
| Classification systems | ABO and Rh |
What blood type is rarest?
Among the eight common ABO/Rh blood types, AB negative (AB−) is the least prevalent. The American Red Cross defines a blood type as rare when it occurs in fewer than 1 in 1,000 people; extremely rare if fewer than 1 in 10,000 have it (American Red Cross). In the United States, AB− occurs in approximately 0.4% of the population (Healthgrades).
What are the 3 rarest blood types?
Breaking down rarity further, the three least common of the eight standard types typically appear in this order: AB negative is rarest, followed by B negative, and then A negative. Data from the NHS Blood Donation registry in the UK shows AB negative at about 1% of donors, B negative around 2%, and A negative at roughly 6% (NHS Blood Donation).
The bottom line: For practical purposes like blood donation, AB negative is the rarest type you’re likely to encounter, which is why blood banks actively encourage AB-negative donors to give regularly.
Which is better, O+ or AB+?
The question of “better” depends entirely on context—whether you are giving or receiving blood. The American Red Cross explains that O positive is the most common blood type in many populations; in the UK donor registry, it represents about 36% of donors (NHS Blood Donation). People with O positive blood can donate red cells to anyone who is Rh-positive (O+, A+, B+, AB+), making their blood highly versatile for general hospital use.
What is the best blood type?
There is no single “best” blood type; each has unique strengths. O negative is widely described as the universal donor for red cell transfusions, because it lacks A, B, and RhD antigens and is considered safe in emergencies for patients of unknown type (El Camino Health). AB positive, on the other hand, is considered the universal recipient for red blood cells in the ABO/Rh system. People with AB+ have A, B, and RhD antigens and therefore lack anti-A and anti-B antibodies, allowing them to receive red cells from any ABO/Rh donor type (NHS (UK)).
Why this matters: Your blood type’s value depends on the clinical situation. O negative is the emergency room’s most requested type; AB positive patients have the widest range of compatible donors.
Do you get your blood type from mom or dad?
Yes, you inherit your blood type from both parents. The ABO gene is located on chromosome 9, and each parent contributes one allele (A, B, or O) to their child. A and B are codominant, while O is recessive (Medicover Genetics). The Rh factor is determined by separate genes on chromosome 1, with Rh-positive being genetically dominant: inheriting a single Rh+ allele is sufficient for an Rh-positive blood type (Moffitt Cancer Center).
Can two siblings have the same blood type?
Yes, siblings can share the same blood type—but they do not have to. Because each child independently inherits a combination of alleles from their parents, same parents can produce children with different ABO/Rh types. According to Our Blood Institute, siblings do not necessarily share the same blood type; each child’s combination is a fresh draw from the parental gene pool (Our Blood Institute). For example, two type A parents carrying AO genotypes can have an O child—a different blood type from both parents (Medicover Genetics).
“Specifically, your blood type is determined by the ABO gene, which is located on chromosome 9. Each parent contributes one allele to their child.” 혈액형에 대한 자세한 내용은 브론테 자매 안내서에서 확인하실 수 있습니다. 브론테 자매 안내서
— Dr. Mandy O’Leary, blood bank and transfusion medicine specialist, Moffitt Cancer Center (source)
The implication: Blood type inheritance follows standard Mendelian genetics. Sharing a blood type with a sibling is common, but not guaranteed, and a child can have a blood type that neither parent possesses.
What two blood types don’t mix?
In transfusion medicine, incompatibility is based on antigen-antibody reactions, not specific pairings of “bad” types. People with blood group O have no A or B antigens on their red blood cells but have both anti-A and anti-B antibodies in their plasma, which limits the types of blood they can safely receive (NHS (UK)). If someone with type A blood receives type B blood, their anti-B antibodies will attack the donor cells—this is a classic incompatible transfusion.
Can O+ and A+ marry?
Yes, absolutely. Blood type compatibility between partners is not a factor in marriage or romantic relationships. The concern about Rh incompatibility specifically arises during pregnancy: if an Rh-negative mother carries an Rh-positive baby, the mother’s immune system may develop antibodies against the baby’s Rh factor (Testing.com). This can be managed effectively with medical interventions during pregnancy.
The bottom line: For daily life and relationships, blood type has no bearing. For medical contexts, the key rule is simple: compatible transfusions require matching ABO and Rh groups to avoid immune reactions.
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For a more detailed breakdown of blood type rarity and compatibility, see detailed breakdown of blood type rarity and compatibility.
Frequently asked questions about blood types
Is O negative a rare blood type?
O negative blood occurs in about 7% of the U.S. population, making it less common than O positive but far from rare by clinical definition. The American Red Cross classifies a type as rare only when it appears in fewer than 1 in 1,000 people (American Red Cross).
Can two siblings have the same blood type?
Yes, siblings can share the same blood type, but it is not guaranteed. Each child independently inherits a random combination of ABO and Rh alleles from the parents, so same parents can produce children with different blood types (Our Blood Institute).
Does blood group matter in marriage?
No, blood group has no relevance to marriage compatibility. The only medical consideration is Rh factor during pregnancy, which can be managed with standard prenatal care (Testing.com).
What is the most common blood type?
O positive is the most common blood type globally. In the UK donor registry, O positive accounts for about 36% of donors, followed by A positive at 28% (NHS Blood Donation).
What is the best blood type?
There is no objectively “best” blood type. O negative is the universal donor, making it most valuable in emergencies. AB positive is the universal recipient, offering the widest compatibility for receiving blood. Each type has its own clinical role (El Camino Health).