Blood Type FAQ: 20 Common Questions Answered

O-negative (O-) is the universal red-cell donor. It carries no A, B, or Rh-D antigens, so it can be safely transfused to any of the 8 blood types. AB plasma (both AB+ and AB-) is the universal plasma donor - the rules invert for plasma because plasma carries antibodies, not antigens.

AB-positive (AB+) is the universal red-cell recipient. AB-positive people carry A, B, and Rh-D antigens, so their immune system does not produce antibodies against any of these. They can receive red cells from all 8 blood types. O-type patients are the most restricted recipients - they can only receive from O donors.

No. There is no scientific evidence that blood type influences personality, temperament, or character. The belief is a popular cultural phenomenon in Japan (known as 'blood type theory' or 'ketsueki-gata') and South Korea, where ABO type is sometimes used to assess personality compatibility - similar to astrology in Western cultures. Multiple peer-reviewed studies have found no correlation between blood type and personality traits. The American Psychological Association and relevant medical bodies do not endorse this claim.

Very rarely - after a bone marrow or stem cell transplant. The transplanted stem cells produce blood cells matching the donor's blood type, which over weeks and months replaces the recipient's original type. Some cancers or severe infections can temporarily affect blood typing test results, but permanent blood type change in a healthy person is not possible.

No. The 'blood type diet' (popularised by Peter D'Adamo in his 1996 book 'Eat Right 4 Your Type') claims different blood types should eat different foods based on how lectins interact with blood antigens. No rigorous, peer-reviewed clinical evidence supports this claim. A 2013 review published in the American Journal of Clinical Nutrition examined studies of blood type diets and found no evidence of benefit tied to blood type specifically. Any benefits individuals report reflect the general quality of the dietary advice, not blood type matching.

The Rh factor refers to the Rh-D antigen - a specific protein on the surface of red blood cells. If your red cells carry this protein, you are Rh-positive. If they do not, you are Rh-negative. About 85% of people are Rh-positive. The Rh factor is relevant primarily in blood transfusion (Rh-negative people should receive Rh-negative blood to avoid sensitisation) and pregnancy (Rh-negative mothers carrying Rh-positive fetuses may need anti-D prophylaxis).

Yes. Non-invasive prenatal testing can determine fetal Rh type from cell-free fetal DNA (cfDNA) in the mother's blood. This is particularly relevant for Rh-negative mothers - knowing the fetus is Rh-negative can allow anti-D prophylaxis to be withheld. This test is widely available in the UK and many European countries; availability in the US varies by centre. Full fetal ABO typing from cfDNA is not yet standard clinical practice but is technically feasible.

An ABO-incompatible transfusion causes an acute haemolytic transfusion reaction (AHTR). The recipient's antibodies attack the donated red cells, causing them to rupture (haemolyse). Symptoms include fever, chills, back or flank pain, dark urine (haemoglobinuria), and a drop in blood pressure. Severe cases can cause acute kidney failure, disseminated intravascular coagulation (DIC), and death. AHTR is the most serious transfusion complication and is almost always due to clinical error (wrong unit given to the wrong patient). Modern blood bank protocols - wristband checking, electronic cross-matching, sample identification - exist to prevent this.

Yes - AB blood type (both AB+ and AB-). AB plasma contains neither anti-A nor anti-B antibodies because AB-type people carry both A and B antigens and produce neither antibody. AB plasma can therefore be given to any patient. This is the inverse of red-cell rules, where O-negative is universal.

Among the 8 main ABO/Rh types, AB-negative (AB-) is the rarest at approximately 0.6% of US donors. Beyond the 8-type system, Rhnull blood (sometimes called golden blood) is estimated in fewer than 50 people worldwide and is the rarest known blood type.

Both. You inherit one ABO allele from your mother and one from your father. Your blood type is the phenotype that results from your two-allele genotype. The Rh factor follows the same pattern - one D or d allele from each parent. Neither parent contributes more to blood type than the other.

No - under standard genetics. O-type individuals have the genotype ii (two recessive i alleles). They have no A or B allele to pass to a child. An AB child requires one A allele from one parent and one B allele from the other. The only theoretical exception involves the Bombay phenotype, which is vanishingly rare.

The possible types depend on the A-positive parent's genotype. If the A+ parent is AA (homozygous A), all children will be A+ or A-. If the A+ parent is AO (heterozygous), children could be A+ or A- or O+ or O-. The O- parent is always OO for ABO, and dd for Rh - they can only contribute O and d alleles. Use our inheritance calculator for specific combinations.

Yes, if the parents are heterozygous. For example, two A+ parents who are both AO and Dd can have children who are A+, A-, O+, or O-. The blood type you can phenotypically observe (A+) does not reveal your full genotype. Hidden recessive alleles (O, d) can be passed to children.

Early in the pandemic (2020), observational studies suggested possible associations between ABO blood type and COVID-19 risk. Some studies found type A individuals had slightly higher risk of severe disease; type O individuals slightly lower risk. However, these associations were small, inconsistent across studies, and confounded by many factors. As of 2026, there is no clinical guidance based on blood type for COVID-19 management. Vaccination remains the key protective measure regardless of blood type.

O-positive (O+) is the most common blood type in the US, found in approximately 37.4% of donors. A-positive (A+) is second at about 35.7%. The two O-positive and A-positive types together account for nearly three-quarters of the US blood donor population.

Yes, in a genuine life-threatening emergency where Rh-negative blood is unavailable. The risk is Rh sensitisation - the Rh-negative person may develop anti-D antibodies. For men and post-menopausal women, this is a manageable one-time risk. For women of childbearing age, it is a more significant concern because anti-D antibodies could affect future pregnancies. Clinicians weigh this risk against the immediate threat to life.

O-positive is sometimes called the universal donor for Rh-positive patients, covering about 84% of the population. However, the true universal donor is O-negative, which can be given to all 8 blood types including Rh-negative individuals. O-positive cannot be given to Rh-negative recipients without risking sensitisation.

A haemolytic transfusion reaction occurs when transfused red cells are attacked and destroyed by the recipient's antibodies. In the acute form (AHTR), this happens immediately and can be life-threatening. In the delayed form (DHTR), it occurs 3-10 days after transfusion due to a secondary immune response. Symptoms of DHTR include declining haemoglobin, jaundice, and fever. Both forms require immediate medical assessment. Strict patient identification and blood bank protocols minimise the risk.

The most reliable sources for blood type information are the American Red Cross (redcrossblood.org), Cleveland Clinic, American Society of Hematology (hematology.org), UNOS/OPTN for transplant, and the National Kidney Foundation. This site cites these sources throughout. See our Sources page for a full annotated reference list.