Blood Types: Rarity, Compatibility, and Health Links

Blood types are more than just labels — they determine who can donate to whom and may even influence health. In Ireland, the distribution of ABO and Rh groups differs from global averages, with implications for transfusion medicine and emerging research on brain health.

Total blood types: 8 (eight major groups) | Most common blood type worldwide: O+ (approx. 38%) | Rarest blood type: AB- (approx. 1%) | Most common in Ireland: O+ (approx. 47%) | Universal red cell donor: O- | Universal plasma donor: AB

What is a rarest blood type?

Rarest blood type overview

Within the ABO and Rh systems, AB negative (AB-) is the least common blood type worldwide, accounting for roughly 1% of the population. The Medical News Today report notes that the rarest commonly cited blood type is Rhnull, which lacks all Rh antigens, but AB- remains the rarest among the eight major types.

Geographic variation in rarity

Rarity percentages vary by region. For instance, the NIH study on Irish blood group distribution found AB frequency at about 3% in Ireland, while global estimates place AB- below 1%. The Better Health Channel explains that rare blood types are not limited to A, B, AB, O; there are more than 300 blood group variants.

Why AB- is so rare

The rarity of AB- stems from the low probability of inheriting both the A and B alleles and the Rh-negative allele. Since both parents must contribute specific combinations, the phenotype appears infrequently.

“AB negative is the least common blood group in Ireland.” — Irish Blood Transfusion Service

Is O+ a rare blood group?

O+ frequency worldwide

O positive is the most common blood type globally, representing approximately 38% of the population. Data from the American Red Cross confirm that roughly one in three people has O+ blood.

Comparison to other types

Compared to AB- (1%) or B- (2%), O+ is not rare. The Medical News Today article states that O blood groups are the most common across geographic regions.

Why O+ is not considered rare

Rarity is typically defined as a frequency below 5%. With O+ above 30%, it does not qualify. In Ireland, the NIH study reports expected O frequency at 56% and RhD positive at 83%, making O+ the majority.

“A person with O positive blood is the most common type you will encounter.” — American Red Cross

What blood type do most Irish have?

Blood type distribution in Ireland

According to the Irish Blood Transfusion Service, O positive is the most common blood group in Ireland, with approximately 47% of donors. The 2021 NIH study reported ABO phenotype frequencies: A 29.82%, B 12.02%, O 54.95%, AB 3.21%, with RhD-positive at 82.26%.

Comparison with global averages

While O+ is also the global leader, Ireland’s O+ percentage is higher than the global average of ~38%. Rh-negative types are less common in Ireland than in some other populations, but the study shows 17.74% RhD-negative frequency.

Implications for blood donation in Ireland

The prevalence of O+ means that supply is generally stable, but O- is always in high demand because it is the universal donor. The Irish Blood Transfusion Service emphasizes that O- recipients can only receive O- blood, making each O- donor critical.

What two blood types don’t mix?

ABO incompatibility basics

Incompatibility arises when a recipient’s immune system attacks foreign antigens on donor red cells. The Australian Red Cross Lifeblood explains that blood group compatibility depends on both ABO type and RhD factor.

Rh incompatibility

An Rh-negative recipient cannot receive Rh-positive blood without risk of sensitization. The American Red Cross notes that AB+ recipients can receive from all ABO types when Rh is matched.

Transfusion matching rules

• O- is universal red-cell donor – Australian Red Cross Lifeblood
• AB+ is universal recipient – American Red Cross
• O- recipients can only receive O- – Irish Blood Transfusion Service

“AB positive recipients can receive blood from all major ABO blood groups when Rh compatibility is satisfied.” — Irish Blood Transfusion Service

What blood type is prone to ADHD?

Research on blood type and ADHD

Studies have explored links between ABO blood type and attention-deficit/hyperactivity disorder. Scientific American reported that children with O type may have a greater risk, while others suggest A type. A 2025 peer-reviewed review in the Journal of Blood Medicine found overrepresentation of AB in neurodevelopmental disorders.

Potential mechanisms

Some hypotheses involve blood group antigens affecting brain development or immune function, but no mechanism has been confirmed.

Limitations of current studies

The evidence remains associative, not diagnostic. The Scientific American article cautions that associations are weak and not clinically actionable. Blood type alone does not determine ADHD.