Blood types — the familiar A, B, AB, and O designations paired with a positive or negative sign — are far more than a medical curiosity. They determine who can give blood to whom, play a role in pregnancy care, and have fascinated researchers for more than a century. Understanding how blood types work, which ones are common or rare, and how to find out your own type is useful knowledge for anyone.
The classification system rests on two key elements: the ABO system and the Rh factor. Together, they produce eight common blood types — A+, A-, B+, B-, AB+, AB-, O+, and O-. Each type is defined by specific antigens, or markers, on the surface of red blood cells.
What is the most common blood type?
According to Stanford Blood Center, O-positive is the most common blood type in the United States, accounting for roughly 37.4% of the population. A-positive follows closely at 35.7%, making these two types the majority. The full breakdown shows B-positive at 8.5%, O-negative at 6.6%, A-negative at 6.3%, AB-positive at 3.4%, B-negative at 1.5%, and AB-negative at just 0.6%.
Blood Type Basics
Four main blood groups (A, B, AB, O) plus Rh factor create 8 common types. Determined by antigens on red blood cells.
Most Common Types
O positive is the most common blood type worldwide (≈38% of population). O negative is the universal donor.
Rarest Types
AB negative is the rarest (≈1% of population). Rh-null is even rarer, but not covered in all sources.
Blood Type Testing
Blood type can be determined through a simple blood test (ABO and Rh typing). Home kits are available but less reliable.
- O positive is the most common blood type, making up about 38-40% of the global population.
- AB negative is the rarest of the standard types, found in less than 1% of people.
- Blood type is inherited from parents and determined by specific gene combinations.
- Knowing your blood type is essential for safe blood transfusions and pregnancy care (Rh factor).
- Blood type tests are quick, inexpensive, and available through clinics, blood drives, and at-home kits.
| Key Blood Type Facts at a Glance | |
|---|---|
| Number of main blood types | 8 (A+, A−, B+, B−, AB+, AB−, O+, O−) |
| Most common blood type | O positive (≈38-40%) |
| Rarest blood type (standard) | AB negative (less than 1%) |
| Universal donor | O negative (can donate to all) |
| Universal recipient | AB positive (can receive from all) |
| Discoverer of ABO system | Karl Landsteiner (1901) |
| Blood type test method | ABO and Rh typing using antibody-based agglutination |
What is the rarest blood type?
AB-negative is the rarest among the eight common blood types in the United States, found in only about 0.6% of the population. Other relatively rare types include B-negative at 1.5% and AB-positive at 3.4%.
Beyond the standard ABO/Rh classification, a condition called Rhnull — sometimes called “golden blood” — is extraordinarily rare, occurring in roughly 1 in 6 million people. Individuals with Rhnull lack all Rh antigens, making their blood valuable for research and specialized transfusions but also difficult to match in an emergency.
What are the 3 rarest blood types?
Among the eight common types, the three rarest in the U.S. based on frequency are AB-negative (0.6%), B-negative (1.5%), and AB-positive (3.4%). Frequencies may vary by geographic region and ethnicity.
“There are eight main blood types but some are rarer than others.”
UK Blood Donation (blood.co.uk)
What is A positive blood type?
A-positive is one of the two most common blood types in the U.S., representing about 35.7% of the population. People with A-positive blood have A antigens on their red blood cells and are Rh-positive, meaning they carry the Rh D antigen.
A-positive recipients can receive blood from A-positive, A-negative, O-positive, and O-negative donors. They can donate to A-positive and AB-positive recipients.
What is A negative blood type?
A-negative is less common, found in about 6.3% of the U.S. population. Because it is Rh-negative, A-negative blood can be given to both Rh-negative and Rh-positive recipients in many cases. A-negative individuals can receive red blood cells from A-negative and O-negative donors.
What is O blood type?
Blood type O is unique because it lacks both A and B antigens. O-positive is the most common blood type overall, but O-negative holds special status as the universal donor for red blood cells.
The American Red Cross notes that O-negative red blood cells can be given to patients of any blood type because they contain no A, B, or Rh D antigens that could provoke an immune reaction. This makes O-negative critically important for emergency transfusions when a patient’s blood type is unknown.
O-positive blood type
O-positive is the most common type in the U.S. at 37.4%. It can only be given to Rh-positive recipients — A+, B+, AB+, and O+. People with O-positive blood are in high demand for donations because their type is so prevalent.
O-negative blood type
O-negative is present in about 6.6% of the U.S. population. Despite its relative rarity, it is the standard choice for emergency transfusions and is often used for trauma patients, premature infants, and patients with weakened immune systems. O-negative donors are always needed.
O-negative is the universal red cell donor, meaning it can be transfused to patients of any blood type. This is because it lacks A, B, and Rh D antigens. AB-positive is the universal recipient for red blood cells, as it has no antibodies against A, B, or Rh.
Blood type test
A blood type test is a straightforward procedure that determines your ABO group and Rh status. It is often performed before a blood transfusion, during pregnancy or when planning pregnancy, before surgery, before organ donation, or as part of a routine blood donation screening.
How to find out your blood type
There are several reliable ways to learn your blood type:
- Ask your doctor for a blood type test, which is a simple lab procedure.
- Donate blood — donation centers typically test and provide your blood type afterward.
- Check prior medical records, since blood typing is often done before surgery, transfusion, or pregnancy care.
While at-home blood type test kits are available, clinical lab results are considered the gold standard for accuracy. For any medical decision — including transfusion or pregnancy care — a professionally administered test is recommended.
Blood type is inherited from both parents, with the ABO gene located on chromosome 9. The combination of parental alleles determines the child’s type. It is worth noting that ABO typing alone is not sufficient to prove or disprove parentage, as many different combinations can produce the same blood type.
Rh factor matters during pregnancy because a baby may inherit Rh-positive blood from either parent. If the pregnant person is Rh-negative and the fetus is Rh-positive, the mother can develop antibodies against fetal red blood cells — a condition known as Rh incompatibility. To prevent complications, clinicians commonly administer Rh immunoglobulin (RhIg) during pregnancy when indicated.
History of blood types: A timeline
- 1901 — Karl Landsteiner discovers the ABO blood group system, identifying A, B, and O groups.
- 1902 — Landsteiner’s colleagues discover the AB group, completing the ABO system.
- 1937 — Landsteiner and Alexander Wiener discover the Rh factor (Rhesus) system.
- 1940s–50s — Blood typing becomes routine for transfusions and pregnancy care.
- 1960s — Modern blood bank systems and donor screening develop.
What is known and what remains unclear about blood types
| Established information | Information that remains unclear or is not supported by evidence |
|---|---|
| Blood type is genetically determined and does not change over a person’s lifetime (except rare bone marrow transplants). | Claims linking blood type to personality or diet are not scientifically supported. |
| ABO and Rh systems are well-understood and clinically validated. | Rare subtypes (e.g., Rhnull, Bombay phenotype) exist but are extremely rare and not part of standard testing. |
| O negative is safe as a universal donor for red blood cells. | Home blood type test kits can be inaccurate; clinical lab results are the gold standard. |
Why blood type knowledge matters
Knowing your blood type is not just for emergencies. It is essential for safe blood transfusions, organ transplants, and managing Rh incompatibility during pregnancy. Ethnic and geographic variations affect blood type frequencies — for example, O blood type is more common in indigenous populations of Central and South America, while AB is less common globally. Public education about blood types supports donation efforts and helps ensure an adequate supply of rare types for patients who need them.
What trusted sources say about blood types
“There are 4 main blood groups (types of blood) – A, B, AB and O. Your blood group is determined by the genes you inherit from your parents.”
NHS
“There are four major blood groups determined by the presence or absence of two antigens, A and B, on the surface of red blood cells.”
American Red Cross
Summary: What to do if you don’t know your blood type
If you don’t know your blood type, consider donating blood — they will test it for you at no charge — or request a blood type test from your healthcare provider. At-home kits are available, but results should be confirmed clinically for any medical decisions. Knowing your blood type is a small piece of information that can make a significant difference in a medical emergency. For related health topics, read about Low Blood Pressure Causes – Symptoms, Dangerous Levels and Treatment and Signs of Ectopic Pregnancy – Early Symptoms and When to Act.
Frequently Asked Questions
Can your blood type change?
No, blood type is genetically fixed. The only rare exceptions are after a bone marrow transplant or if someone has a condition like leukemia that affects blood cells.
What is the Rh factor?
The Rh factor is a protein on red blood cells. Rh-positive (+) means you have the protein; Rh-negative (−) means you don’t. It’s important for pregnancy and transfusions.
Is there a connection between blood type and diet?
The blood type diet theory has not been supported by scientific evidence. Most health experts advise a balanced diet regardless of blood type.
What is the universal blood type for donations?
O negative is considered the universal donor for red blood cells because it can be given to patients of any blood type. AB positive is the universal recipient.
How do I find out my blood type?
You can learn your blood type through a blood donation, a clinical lab test, or a home testing kit. Donation services typically test and provide your type for free.
What is the most common blood type in the world?
O positive is the most common blood type globally, though exact percentages vary by region and ethnicity.
What is the rarest blood type in the world?
Among standard types, AB negative is rarest. Rhnull, known as golden blood, is even rarer but does not appear in standard population statistics.
Can two parents with type O have a child with type A?
No. Two O parents can only have children with type O blood, since O is a recessive trait.
Is blood type testing painful?
No. It is a standard blood draw, similar to a routine blood test. Some donation centers use a finger-prick method for quick typing.
Why do blood types matter for pregnancy?
If a pregnant person is Rh-negative and the fetus is Rh-positive, the mother’s immune system may produce antibodies against the baby’s blood. This is prevented with Rh immunoglobulin injections.
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Grace Hollis is Film & Music Editor at StoryNative.uk, covering film releases, music news, soundtracks and awards season.