A quick glance at a blood donation form or a medical chart reveals a simple letter-and-sign code: A+, O−, AB−. That two-character label encodes a set of biological rules that determine who can receive your blood in an emergency, which transfusions are safe, and how your body’s immune system sees other people’s red cells. This article answers the most Googled questions about blood type — rarity, inheritance, and compatibility — using verified information from national health services and peer-reviewed sources such as the NHS.
Last checked: 2026-07-02
Number of main blood types: 8 (A+, A-, B+, B-, AB+, AB-, O+, O-) · Classification systems: ABO system and Rh system · Rarest blood type: AB-negative · Universal donor: O-negative
How we researched this
Last checked: 2026-07-02.
Sources reviewed: Official blood donation organizations, national health service websites, major medical center patient information, peer-reviewed transfusion medicine publications.
We did not conduct original clinical research, interview hematologists, or visit blood donation centers.
Blood type basics at a glance
- 8 common types: A+, A−, B+, B−, AB+, AB−, O+, O− (Cleveland Clinic)
- ABO system and Rh system — the two main grouping methods used globally (Healthdirect Australia)
- AB-negative — approximately 1% of the population (Medical News Today)
- O-negative — compatible with all ABO and Rh types in emergency red cell transfusions (El Camino Health)
| Item | Detail |
|---|---|
| Most common blood type globally | O-positive — approximately 37% of population |
| Rarest blood type globally | AB-negative — approximately 1% of population |
| Universal donor blood type | O-negative — compatible with all ABO and Rh types |
| Probability two siblings share the same blood type | Approximately 25% chance for full siblings under simple inheritance, varies with parent genotypes |
| Number of main blood types | 8 (A+, A−, B+, B−, AB+, AB−, O+, O−) |
| Classification systems | ABO system and Rh system |
| Rarest blood type | AB-negative |
| Universal donor | O-negative |
What blood type is the rarest?
Among the eight common ABO/Rh blood types, AB-negative is consistently cited as the least frequent, occurring in roughly 1% of people globally, according to Medical News Today. The reasons are genetic: both the AB allele (which produces A and B antigens simultaneously) and the Rh-negative allele must be inherited, and each is rare in many populations.
Why is AB-negative considered rare?
To be AB-negative, a person must inherit one A allele and one B allele from their parents (producing the AB blood group) and also inherit two copies of the recessive Rh-negative allele. The combination of the uncommon AB genotype with the even less common Rh-negative genotype produces a very small subset of the population. Rarity varies by ethnicity and region, but AB-negative remains the least prevalent across most datasets.
How is rarity measured across populations?
Blood type frequency is not uniform worldwide. For instance, O-positive blood is the most common type across many populations, while AB-negative stays rare nearly everywhere. The International Society of Blood Transfusion recognizes 33 blood group systems and over 300 red cell antigens, meaning that within those systems, some types are so uncommon that they are managed through dedicated rare-donor registries.
The bottom line: AB-negative is the rarest of the eight common ABO/Rh types, but other blood group systems contain hundreds of additional antigens that can be even more unusual.
How are blood types inherited?
Blood type is determined by the genes you inherit from your biological parents, as explained by the NHS. Each parent contributes one ABO allele. A and B are codominant (both can express), while O is recessive. The combination of the two alleles produces one of four blood groups: A (AA or AO), B (BB or BO), AB (AB), or O (OO).
Do you get your blood type from mom or dad?
You receive one ABO allele from each parent, so both contribute equally to your blood group. The Rh factor (positive or negative) follows a separate inheritance pattern: Rh-positive is dominant over Rh-negative. If both parents are Rh-negative (dd), their child will always be Rh-negative. If at least one parent is Rh-positive (DD or Dd), the child could be positive or negative depending on the second parent’s genotype. This means siblings can have different blood types, as the process resembles a genetic coin flip for each child.
Can two siblings have the same blood type?
Yes, but it is not guaranteed. For full siblings, the chance of sharing the same ABO and Rh type is approximately 25% under simple inheritance patterns, though this probability shifts with the parents’ specific genotypes. For example, two O-positive parents can only have O-positive or O-negative children, but two parents who are both AO and Rh-positive (Dd) could have children with A or O blood, each either positive or negative. The exact distribution depends entirely on the parental alleles.
Why this matters: Knowing the genetics behind blood type inheritance explains why family members often differ and why blood type cannot be guessed from a parent’s type alone.
“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 information for the public
Which blood types are compatible?
Transfusion compatibility is governed by the immune system’s reaction to antigens on donor red cells. The Cleveland Clinic explains that blood typing is required before transfusions to avoid dangerous immune reactions. Type A blood has A antigens and anti-B antibodies; type B has B antigens and anti-A antibodies; type AB has both antigens and no antibodies; type O has no A or B antigens but both antibodies.
What two blood types don’t mix?
Mixing incompatible blood types — for example, giving type A red cells to a type B recipient — can trigger an acute hemolytic transfusion reaction. The recipient’s anti-A antibodies attack the donor’s A antigens, causing clumping and destruction of red cells, a potentially life-threatening complication described by Healthdirect Australia. O-negative is the universal donor for red cells because it lacks A, B, and RhD antigens. AB-positive is the universal recipient because it has no anti-A or anti-B antibodies and carries the RhD antigen.
Can A-positive and O-positive have a baby?
Yes. A-positive and O-positive parents can have a child with any ABO type depending on the A-positive parent’s genotype (AA or AO). If the A-positive parent is AO, the child has a 50% chance of type A and a 50% chance of type O. Rh factors follow the standard dominant/recessive pattern. The main pregnancy concern arises when an Rh-negative mother carries an Rh-positive baby, which can lead to Rh incompatibility and hemolytic disease of the newborn if not managed with preventative treatment.
The pattern: Compatibility is a two-way street: your antibodies determine which donor blood you can safely receive, and your antigens determine whose bodies can safely receive yours.
Why is O+ so special?
O-positive is the most common blood type in the ABO/Rh system, representing about 37% of people in many populations, according to Medical News Today. This high frequency makes O+ blood the most-used type in hospitals, and its Rh-positive status allows it to be given to any Rh-positive recipient.
Is O-negative a rare blood type?
O-negative is considerably rarer than O-positive, occurring in approximately 7% of the population. Despite its lower frequency, O-negative is the universal donor for red cells because it lacks A, B, and RhD antigens, as described by El Camino Health. This makes it the default blood type issued in trauma and emergency settings when there is no time to cross-match. Blood services actively recruit O-negative donors to maintain adequate emergency supplies.
Which is better, O+ or AB+?
Neither is inherently “better.” O+ has wider donation reach (any Rh-positive recipient) and is the most common type, so it is always in high demand. AB+ has the narrowest donation reach (only other AB+ recipients) but the broadest receiving capacity, as AB+ individuals can accept red cells from any ABO and Rh type. The clinical “value” of a blood type depends entirely on the context: hospital inventory needs, recipient matching, and emergency availability.
The implication: The most common type (O+) keeps hospitals running day-to-day, while the rare universal donor (O−) is the emergency safety net.
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Understanding the distribution of ABO and Rh factors is crucial, and a detailed explanation of blood group rarity provides a helpful breakdown of which types are most and least common globally.
Frequently asked questions about blood types
What blood type are most Irish people?
O-positive is the most common blood type in Ireland, consistent with many Western European populations. Irish blood services report that approximately 47% of the population is O-positive, followed by A-positive at around 26%. AB-negative remains the rarest at roughly 1%.
What was Jesus’s blood type?
There is no reliable scientific or historical evidence to determine any individual’s blood type from 2,000 years ago. Blood typing requires a fresh or preserved sample with intact red cells, which does not exist for historical figures. Speculative claims about Jesus’s blood type are not supported by peer-reviewed research.
What is inherited from father only?
No blood type trait is inherited exclusively from the father. Each parent contributes one ABO allele and one Rh allele. However, the Y chromosome — which determines male sex — is passed only from father to son. Blood type itself is not sex-linked.
Can blood types influence pregnancy risks?
Yes. Rh incompatibility occurs when an Rh-negative mother carries an Rh-positive baby. The mother’s immune system can produce antibodies against the baby’s RhD antigen, potentially causing hemolytic disease in subsequent pregnancies if untreated. Routine antenatal screening and anti-D immunoglobulin injections prevent this complication, as established by national health guidelines.
What are the 3 rarest blood types?
Among the eight common ABO/Rh types, the three rarest are AB-negative (~1%), B-negative (~2%), and AB-positive (~3%). Frequencies vary by population. Beyond these, hundreds of rare antigens exist within other blood group systems managed through international rare-donor registries.
How is blood type determined?
A blood sample is mixed with reagents containing anti-A, anti-B, and anti-RhD antibodies. Visible clumping (agglutination) indicates the presence of the corresponding antigen on the red cells. This test, described by Grifols, is the standard method used in clinical laboratories worldwide.
Can your blood type change?
In the vast majority of people, blood type is fixed for life. Very rare exceptions occur after a bone marrow or stem cell transplant, when the recipient’s blood-forming cells are replaced by the donor’s, potentially changing the ABO and Rh type. There is no evidence that diet, lifestyle, or aging alters blood type.
Do blood types affect personality?
There is no robust scientific evidence linking ABO blood type to personality traits. Popular culture in some countries, particularly Japan and South Korea, associates blood types with character, but peer-reviewed studies have not replicated these claims under controlled conditions.
“Blood types classify blood so healthcare providers can determine whether one person’s blood is compatible or incompatible with someone else’s blood.”
The bottom line: “Blood types classify blood so healthcare providers can determine whether one person’s blood is compatible or incompatible with someone else’s blood.”— Cleveland Clinic patient information