Blood Types: Rarest, Most Common, and Irish Distribution

Few things are as personal as your blood type — it’s a detail you might not think about until you need a transfusion or see a peculiar cultural reference. In Ireland, O positive blood appears in 47% of the population, a figure that shapes donor campaigns and hospital stocks. This article breaks down the rarest types, the most common ones, and what compatibility really means.

Total main blood types (ABO system): 4 ·
Total blood types (including Rh factor): 8 ·
Most common blood type in Ireland: O+ ·
Rarest blood type globally: AB- ·
Percentage of Irish population with O+: 47% ·
Percentage of donors with AB- in UK: 1%

The eight blood types follow a consistent compatibility matrix.

What Is the Rarest Blood Type?

When people ask about the rarest blood type, the answer is almost always AB-negative. Globally, AB- is found in less than 1% of donors, according to NHS Blood Donation, the UK’s national blood service. In Ireland, the Irish Blood Transfusion Service (IBTS) confirms that AB- accounts for just 1% of the population.

“AB- is the rarest blood type worldwide, with prevalence under 1%.”

— American Red Cross, humanitarian organisation

Rarest blood type globally

• AB- is the rarest blood type worldwide, with prevalence under 1% (American Red Cross, humanitarian organisation)
• B- is the second rarest, at about 2% of donors (NHS Blood Donation, UK blood service)

Rarest blood type in Ireland

• AB- is also the rarest in Ireland, at 1% of the population (Irish Blood Transfusion Service, national blood authority)
• B- is rare in Ireland as well, at roughly 2% (Irish Blood Transfusion Service, national blood authority)

The implication: rarity is relative. AB- may be scarce globally, but in Ireland the supply is especially tight because only 3% of the eligible population donates blood, as IBTS reports. That means every AB- donor is disproportionately valuable.

Is O+ a Rare Blood Group?

O+ is the most common blood type in Ireland, at 47% of the population. Globally, it represents about 35% of donors, according to NHS Blood Donation. That makes O+ anything but rare.

“O positive is the most common blood group in Ireland at 47% of the population.”

— Irish Blood Transfusion Service, national blood authority

O+ blood type prevalence

• O+ is the most common blood type globally, found in about 35% of donors (NHS Blood Donation, UK blood service)
• In Ireland, O+ prevalence is even higher at 47% (Irish Blood Transfusion Service, national blood authority)

Why O+ is not considered rare

A blood type is considered rare when it is difficult to find for patients who need it. Since O+ is common in every population studied, it does not meet the rarity threshold. However, O+ is still in high demand because it can be given to anyone with a positive Rh factor.

What Are the 3 Most Common Blood Types?

The three most common blood types globally are O+, A+, and B+. Their order shifts slightly by region, but the top three remain consistent.

Most common blood types globally

• O+ – approximately 35% of donors (NHS Blood Donation, UK blood service)
• A+ – approximately 30% of donors (American Red Cross, humanitarian organisation)
• B+ – approximately 10% of donors (American Red Cross, humanitarian organisation)

Most common blood types in Ireland

• O+ – 47% of the population (Irish Blood Transfusion Service, national blood authority)
• A+ – about 30% (estimated from IBTS data on A prevalence)
• B+ – about 10% (estimated from IBTS data on B prevalence)

The pattern: Ireland’s O+ percentage is significantly higher than the global average, a quirk of genetic history that affects donor recruitment.

What Blood Type Do Most Irish People Have?

The answer is O positive. At 47%, it is the dominant blood type in Ireland, far ahead of A+ at roughly 30% and B+ at about 10%.

Blood type distribution in Ireland

• O+ – 47% (Irish Blood Transfusion Service, national blood authority)
• A+ – 30% (estimated from 2021 NIH study: overall A 29.82%) (PMC / NIH, academic research database)
• B+ – 12% (from same NIH study: B 12.02%)
• AB+ – 3% (from same NIH study: AB 3.21%)

Comparison with global averages

Globally, O+ averages about 35%. Ireland’s 47% is well above that, while the global O- rate of 7% is comparable to Ireland’s estimated 5–6%. This higher O concentration, especially in the west, reflects centuries of relative genetic isolation.

“People in the West of Ireland are predominantly blood group O.”

— Irish Blood Transfusion Service, national blood authority

What Two Blood Types Don’t Mix?

Any transfusion that introduces antigens the recipient’s immune system will attack is dangerous. The most common mismatches involve A and B antigens and the Rh factor.

Incompatible blood type combinations

• Blood type A cannot receive from type B or AB.
• Blood type B cannot receive from type A or AB.
• Blood type AB can receive from all (universal recipient).
• Blood type O can only receive from O.

ABO compatibility rules

The ABO system is based on A and B antigens. If the recipient has antibodies against an antigen, mixing causes agglutination — a potentially fatal reaction. For example, a person with type A blood has anti-B antibodies, so B blood is incompatible.

Rh factor compatibility

Rh factor adds another layer. Rh-negative individuals can only receive Rh-negative blood. Rh-positive individuals can accept both, but Rh-negative blood is always preferred for emergencies to avoid sensitisation.