Who is the universal blood donor?

O-negative (O−) is the universal red-cell donor. O− blood carries neither A nor B antigens nor the Rh-D antigen, so a recipient's immune system has nothing to react against. That's why O− is the type emergency rooms reach for when there's no time to determine a patient's blood type. For plasma, the universal donor is the opposite — AB plasma, because it carries no anti-A or anti-B antibodies.

Who is the universal blood recipient?

AB-positive (AB+) is the universal red-cell recipient. People with AB+ blood already have A, B, and Rh-D antigens, so their immune system won't attack donor cells carrying any of them — they can safely receive red cells from any ABO/Rh type. For plasma it's the reverse: type O recipients can take plasma from anyone.

Can O-negative people receive any blood type?

No — this is the common misconception. O− is the universal donor, but O− people can only receive O− blood. Their immune system would react against the A, B, or Rh-D antigens on any other type. Universal donor and universal recipient are opposite roles: O− gives to all, AB+ receives from all.

What is the rarest blood type?

Among the eight common ABO/Rh types, AB− is the rarest in most populations (roughly 0.6% in the US), followed by B− (~1.5%). The truly rarest types are outside the ABO/Rh system entirely — for example the Rh-null (“golden blood”) phenotype, which lacks all Rh antigens and is described in research as the rarest blood type in the world.

Does blood type matter for pregnancy?

Yes, specifically the Rh factor. If a pregnant person is Rh-negative and carries an Rh-positive baby, their immune system can form antibodies against the baby's red cells — a risk called Rh incompatibility or sensitization. It rarely affects a first pregnancy but can endanger later ones. The standard prevention is an anti-D immunoglobulin injection (RhoGAM is one US brand). It is usually given around 28 weeks, within 72 hours after birth if the baby is Rh-positive, and after events such as bleeding, miscarriage or a bump to the tummy. ABO incompatibility between mother and baby can also occur but is usually milder.

Can a blood test prove or rule out paternity?

Blood type can never confirm parentage, and it is not a reliable way to rule someone out either. Charts say two O parents can only have O children, but the American Red Cross notes that testing issues can cause exceptions, and that ABO typing is not enough to prove or disprove who a parent is. Only DNA testing through an accredited lab gives reliable parentage answers — blood type alone is not a paternity test.

Blood Type Compatibility Chart

Who can donate to whom — and who’s the universal donor and recipient. Useful for understanding transfusions, donations, and family blood-type questions.

The full compatibility table

Type	Can give to	Can receive from	~ US population
O-	Everyone (universal donor)	O-	~7%
O+	O+, A+, B+, AB+	O-, O+	~37%
A-	A-, A+, AB-, AB+	A-, O-	~6%
A+	A+, AB+	A-, A+, O-, O+	~36%
B-	B-, B+, AB-, AB+	B-, O-	~1.5%
B+	B+, AB+	B-, B+, O-, O+	~8%
AB-	AB-, AB+	AB-, A-, B-, O-	~0.6%
AB+	AB+ only	Everyone (universal recipient)	~3.4%

Why the rules look like this

Blood compatibility is about which antigens your body already has and which antibodies it would make against foreign antigens.

Type O has neither A nor B antigens, so anyone can receive it without an immune reaction. That’s why O- is the universal donor.
Type AB already has both A and B antigens, so antibodies against A or B won’t fire. AB+ can therefore receive from anyone — the universal recipient.
Rh-negative recipients can’t safely receive Rh-positive blood without risk of forming anti-D antibodies, which is especially important for women of reproductive age (it can complicate future pregnancies).

Plasma is the inverse

Plasma carries the antibodies rather than the antigens, so plasma compatibility runs the opposite way: AB plasma is the universal donor (no anti-A or anti-B antibodies); O plasma can only go to O recipients. If you donate plasma specifically, your blood centre will know your compatibility.

Pregnancy and Rh

If a pregnant person is Rh-negative and the baby is Rh-positive (inherited from the father), the mother’s immune system can make antibodies that affect future pregnancies. The standard preventive treatment is an injection of RhoGAM (anti-D immunoglobulin). It is usually given at around 28 weeks, within 72 hours after birth if the baby is Rh-positive, and after events such as bleeding, miscarriage or a bump to the tummy. It stops antibodies forming, but it cannot help once they have already formed.

To see which blood types a baby could inherit from two parents, our Blood Type Calculator lays out the possibilities.

How common is each blood type?

Blood type frequencies vary a lot by ancestry and country, but in the United States the rough breakdown is:

O+ — about 37% (the most common type)
A+ — about 36%
B+ — about 8%
O− — about 7% (in high demand as the universal donor)
A− — about 6%
AB+ — about 3.4% (the universal red-cell recipient)
B− — about 1.5%
AB− — about 0.6% (the rarest of the eight)

Because O− is both rare and universally transfusable, it is in constant high demand: in the UK about 8% of people are O negative, but it makes up around 13% of hospital requests for red cells. If you’re O−, you are an unusually valuable donor — a single donation can be given to anyone, including newborns in emergencies.

What actually happens in an incompatible transfusion

If someone receives red cells carrying an antigen their immune system doesn’t recognise, pre-formed antibodies bind to the donor cells and trigger a haemolytic transfusion reaction — the donor cells are destroyed. Symptoms range from fever and chills to dangerous drops in blood pressure and kidney injury. This is why hospitals cross-match blood carefully before any non-emergency transfusion, and why the universal-donor/universal-recipient rules exist as a safety fallback when there’s no time to type a patient.

The Rh-D antigen behaves a little differently: an Rh-negative person doesn’t usually have anti-D antibodies until they’ve been exposed to Rh-positive blood once (through transfusion or pregnancy). After that first exposure they become “sensitised,” and a later exposure can trigger a reaction — which is the whole reason RhoGAM exists for pregnancy.

Why blood-type myths get traction

Type-based diets and personality theories have circulated for decades — the “blood type diet,” or the Japanese ketsueki-gata belief that type predicts personality. No reputable study has confirmed any link between blood type and personality, diet effectiveness, or weight loss. (Blood type is linked to a few health risks — for example, people who are not type O have a somewhat higher risk of blood clots.) A large 2014 study in PLOS ONE tested the blood-type diet directly and found that any benefits people experienced were unrelated to their actual blood type. The biology of ABO and Rh — which antigens sit on your red cells — is well-mapped and genuinely matters for transfusion and pregnancy. The lifestyle claims layered on top of it are not supported.

Frequently asked questions

Who is the universal blood donor?: O-negative (O−) is the universal red-cell donor. O− blood carries neither A nor B antigens nor the Rh-D antigen, so a recipient's immune system has nothing to react against. That's why O− is the type emergency rooms reach for when there's no time to determine a patient's blood type. For plasma, the universal donor is the opposite — AB plasma, because it carries no anti-A or anti-B antibodies.
Who is the universal blood recipient?: AB-positive (AB+) is the universal red-cell recipient. People with AB+ blood already have A, B, and Rh-D antigens, so their immune system won't attack donor cells carrying any of them — they can safely receive red cells from any ABO/Rh type. For plasma it's the reverse: type O recipients can take plasma from anyone.
Can O-negative people receive any blood type?: No — this is the common misconception. O− is the universal donor, but O− people can only receive O− blood. Their immune system would react against the A, B, or Rh-D antigens on any other type. Universal donor and universal recipient are opposite roles: O− gives to all, AB+ receives from all.
What is the rarest blood type?: Among the eight common ABO/Rh types, AB− is the rarest in most populations (roughly 0.6% in the US), followed by B− (~1.5%). The truly rarest types are outside the ABO/Rh system entirely — for example the Rh-null (“golden blood”) phenotype, which lacks all Rh antigens and is described in research as the rarest blood type in the world.
Does blood type matter for pregnancy?: Yes, specifically the Rh factor. If a pregnant person is Rh-negative and carries an Rh-positive baby, their immune system can form antibodies against the baby's red cells — a risk called Rh incompatibility or sensitization. It rarely affects a first pregnancy but can endanger later ones. The standard prevention is an anti-D immunoglobulin injection (RhoGAM is one US brand). It is usually given around 28 weeks, within 72 hours after birth if the baby is Rh-positive, and after events such as bleeding, miscarriage or a bump to the tummy. ABO incompatibility between mother and baby can also occur but is usually milder.
Can a blood test prove or rule out paternity?: Blood type can never confirm parentage, and it is not a reliable way to rule someone out either. Charts say two O parents can only have O children, but the American Red Cross notes that testing issues can cause exceptions, and that ABO typing is not enough to prove or disprove who a parent is. Only DNA testing through an accredited lab gives reliable parentage answers — blood type alone is not a paternity test.

Related tools

Blood Type Calculator — predict a child’s blood type from both parents

Sources

Checked against the following public guidance on September 26, 2026. Advice can differ between countries; your doctor, nurse or midwife can apply it to you.