- 01Red cells and plasma follow opposite compatibility rules
Your blood type describes the antigens (surface markers) on your red cells and the antibodies in your plasma. For a red cell transfusion, the donor's antigens are what matter, so O negative, with no A, B or RhD antigen, is the universal red cell donor. For plasma, the donor's antibodies are what matter, so AB, with no anti-A or anti-B, is the universal plasma donor. Every line of a compatibility chart follows from that one reversal.
- 02The eight types are where typing starts
ABO and RhD set the eight common types, but red cells carry more than 300 other known antigens. Those explain the rarest types such as Rh null and the Bombay phenotype, and they are why a lab still crossmatches donor and patient blood before a planned transfusion, even when the chart says the types are compatible.
Your blood type is a short description of two things: which markers sit on the surface of your red blood cells, and which antibodies float in your plasma, the liquid part of blood. Two systems set the eight common types. The ABO system gives the letter (A, B, AB or O), and the Rh system gives the plus or minus 6. Every rule about who can give blood to whom follows from those two facts, and so does almost everything people search about blood types, from the rarest type to why Rh matters in pregnancy.
What are the blood types?
Blood types are defined by antigens, the molecules made of sugars and proteins that coat the surface of a red blood cell 5. In the ABO system there are two antigens, A and B. Group A red cells carry A, group B red cells carry B, group AB red cells carry both, and group O red cells carry neither 6.
Your plasma carries antibodies against whichever ABO antigen your own cells lack. Group A blood has anti-B antibodies, group B has anti-A, group O has both anti-A and anti-B, and group AB has neither 6. Antibodies are the body's recognition system for anything foreign, which is why receiving red cells from the wrong ABO group can be life-threatening, because the recipient's antibodies attack the donor cells 6 7.
The Rh system adds the plus or minus. If your red cells carry the RhD antigen, your blood type is positive; if they lack it, your blood type is negative 5. Rh negative people do not carry anti-D antibodies by default. They can make them after contact with RhD positive blood, through a transfusion or a pregnancy 9, and that difference drives the pregnancy story further down this page. Combine four ABO groups with two Rh options and you get the eight common blood types 6.
Blood type chart
This blood type chart lists all eight types with what each carries and the red cell transfusion rules that follow. Read across a row: the antigens column says what your cells carry, the antibodies column says what your plasma will attack, and the last two columns are the consequence.
| Blood type | Antigens on red cells | Antibodies in plasma | Can give red cells to | Can receive red cells from |
|---|---|---|---|---|
| O negative (O-) | None | Anti-A and anti-B | All eight types | O- only |
| O positive (O+) | RhD | Anti-A and anti-B | O+, A+, B+, AB+ | O+, O- |
| A negative (A-) | A | Anti-B | A-, A+, AB-, AB+ | A-, O- |
| A positive (A+) | A and RhD | Anti-B | A+, AB+ | A+, A-, O+, O- |
| B negative (B-) | B | Anti-A | B-, B+, AB-, AB+ | B-, O- |
| B positive (B+) | B and RhD | Anti-A | B+, AB+ | B+, B-, O+, O- |
| AB negative (AB-) | A and B | None | AB-, AB+ | AB-, A-, B-, O- |
| AB positive (AB+) | A, B and RhD | None | AB+ only | All eight types |
The chart describes red cell transfusions, the most common kind, and the rules come from the American Red Cross and MedlinePlus 2 7. Two patterns are worth holding on to. Rh positive people can receive Rh positive or Rh negative red cells, while Rh negative people receive Rh negative red cells 7. And because every type can receive its own type, a matching donor is always the first choice; the cross-type options exist for shortages and emergencies.
Who can donate to whom?
Red cells and plasma follow opposite rules, and that reversal explains every line of a compatibility chart. For red cells, what matters is the antigens on the donor's cells, because the recipient's antibodies will attack any antigen they do not recognize. For plasma, what matters is the antibodies in the donor's plasma, because those antibodies would attack the recipient's cells. The American Red Cross puts it in one line: the rules for plasma are the reverse 1.
Red cell and plasma compatibility run in opposite directions
Universal donor and universal recipient
The universal red cell donor has type O negative blood and the universal plasma donor has type AB blood 1. O negative red cells lack the A, B and RhD antigens, so in most cases they can be given to anyone, and they are used in emergencies when a patient's type is not yet known 6. AB positive is called the universal recipient because AB positive patients can receive red cells from all blood types 2. AB plasma is in short supply because only about 4 percent of people in the US have type AB (about 3.4 percent AB positive and 0.6 percent AB negative) 3.
The word universal is a shorthand, and it covers ABO and RhD only. Red cells carry more than 300 other known antigens 1, and a person who has made antibodies against one of them, for example after an earlier transfusion or pregnancy, can react to blood that looks compatible on the chart. That is why the lab runs a crossmatch, described below.
Blood types by rarity: how common is each type?
The table below gives the usual US estimates for each blood type. The American Red Cross rounds a little differently (for example, 37 to 38 percent for O positive, 7 percent for O negative, and fewer than 1 percent for AB negative) 1 2, but the order is the same in every source: O positive and A positive together cover more than two in three people, and the AB types are the scarcest.
| Blood type | Share of people in the US | Roughly 1 person in |
|---|---|---|
| O positive | 37.4% | 3 |
| A positive | 35.7% | 3 |
| B positive | 8.5% | 12 |
| O negative | 6.6% | 15 |
| A negative | 6.3% | 16 |
| AB positive | 3.4% | 29 |
| B negative | 1.5% | 67 |
| AB negative | 0.6% | 167 |
These are general estimates for education, drawn from the AABB Technical Manual by Stanford Blood Center 3. Add up the positive types and about 85 percent of people in the US are Rh positive, which matches the Red Cross figure that more than 80 percent of the population has a positive blood type 2. The UK picture is similar: about 85 percent of people are RhD positive and around 48 percent are group O 6.
Blood type by ethnicity and ancestry
Blood type frequencies differ by ancestry, sometimes a lot. The largest US study, covering 3.1 million blood donors at five blood centers who chose their own racial or ethnic category, found the highest share of group O among Hispanic (56.5 percent), North American Indian (54.6 percent) and Black non-Hispanic (50.2 percent) donors 4. The Red Cross puts group O at about 45 percent of white Americans 1.
| Donor group (US) | Group O, any Rh | Rh negative, any ABO type | O negative | B negative |
|---|---|---|---|---|
| White, non-Hispanic | About 45% 1 | 17.3% | 8.0% | 1.8% |
| Hispanic | 56.5% | 7.3% | 3.9% | 0.7% |
| Black, non-Hispanic | 50.2% | 7.1% | 3.6% | 1.3% |
Figures from Garratty and colleagues, 2004, unless marked 4. Asian donors had the lowest shares of O negative (0.7 percent) and B negative (0.4 percent) blood of the four groups 4. The same study found that Asian donors had the highest shares of B positive and AB positive blood, Hispanic donors the highest share of O positive, and white non-Hispanic donors the highest share of Rh negative 4. The Red Cross notes that certain blood types are unique to specific ethnic or racial groups, and that a donation from an African American donor may be the best hope for patients with sickle cell disease, many of whom are of African descent 1.
What is the rarest blood type?
Of the eight common types, AB negative is the rarest. Fewer than 1 percent of people in the US have it, making it the least common blood type among Americans 2, and the AABB-based estimate is 0.6 percent, about 1 person in 167 3. B negative comes next at about 1.5 percent, then AB positive at about 3.4 percent 3.
The rarest blood types sit outside the eight. Red cells carry more than 300 other known antigens, and the American Red Cross counts your blood type as rare if you lack an antigen that 99 percent of people have, and extremely rare if you lack one that 99.99 percent of people have 1. People with these types usually find out through donating or after a transfusion workup, and rare donor registries exist to match them.
Rh null, sometimes called golden blood
In Rh null, the red cells carry no Rh antigens at all 10. It is so rare that a 2026 scoping review of published cases found six eligible reports, describing eight people in total 10. People with Rh null often have a long-term hemolytic anemia, usually moderate, because their red cells break down early, and once they have formed antibodies they can generally receive only Rh null blood, which comes from family members, frozen stores or rare donor registries 10.
The Bombay phenotype
The Bombay blood group, first described in Bombay (now Mumbai) in 1952, lacks the H antigen, the base structure on which the A and B antigens are built. People with it carry no A, B or H antigen on their red cells and have anti-H antibodies in their plasma 11. It is estimated at about 1 in 10,000 people in India and about 1 in 250,000 people of European descent 11. On a quick test of the red cells alone, Bombay blood looks like group O, and giving it ordinary group O blood can cause a severe transfusion reaction. Testing the plasma as well (reverse grouping, with group O cells as a control) catches it 11.
O positive blood type
O positive is the most common blood type, found in about 37 to 38 percent of people in the US 1 2. O positive red cells carry the RhD antigen but no A or B, so they can be given to anyone who is Rh positive: A positive, B positive, AB positive and O positive 2. More than 80 percent of people have a positive blood type and can receive O positive blood, so demand is high, and O types are the first to run out during a shortage 2. People with O positive blood can receive red cells from O positive or O negative donors 7.
O negative blood type
About 7 percent of people in the US have O negative blood 1. O negative red cells lack A, B and RhD, which makes O negative the universal red cell donor and the type emergency teams reach for when there is no time to check a patient's type 1 6. The Red Cross notes it is also the blood needed for immune deficient infants 1. The trade-off is that O negative is the most limited recipient: people with O negative blood can receive red cells from O negative donors only 7.
A, B and AB blood types
A positive is the second most common type, about 1 in 3 people; A positive red cells go to A positive and AB positive patients 2 3. A negative is about 1 in 16 people, and A negative red cells can go to anyone who is A or AB, positive or negative 2. About 9 percent of people are B positive and fewer than 2 percent are B negative; B negative red cells can go to both B and AB patients, while B negative patients receive B negative or O negative blood 2. AB positive is fewer than 4 percent of people and AB negative fewer than 1 percent, and AB blood is the source of universal plasma 2.
How blood typing works
A lab types blood by watching for clumping, called agglutination. In forward typing, your red cells are mixed with reagents containing anti-A and anti-B antibodies, and clumping shows which antigens are present. In back typing, your serum (plasma without the clotting proteins) is mixed with known type A and type B cells, which shows which antibodies you carry. Rh typing uses the same method with anti-D 7. The two halves should agree; when they do not, the lab investigates before calling the type.
| Clumps with anti-A? | Clumps with anti-B? | Clumps with anti-D? | Blood type |
|---|---|---|---|
| Yes | No | Yes | A positive |
| Yes | No | No | A negative |
| No | Yes | Yes | B positive |
| No | Yes | No | B negative |
| Yes | Yes | Yes | AB positive |
| Yes | Yes | No | AB negative |
| No | No | Yes | O positive |
| No | No | No | O negative |
This is the forward typing result, the same pattern taught in anatomy lab blood typing exercises. If the cells do not react with anti-A or anti-B, the blood is group O 6. Back typing then confirms it: group O plasma should clump both A cells and B cells, group AB plasma neither 7.
How to find out your blood type
Blood type is not part of routine bloodwork. A complete blood count counts your red cells and measures hemoglobin and the red blood cell count, but it does not type those cells, and in the UK, family doctors do not routinely check blood group 6. These are the usual ways to find out.
- 1Give bloodBlood services type every donation and tell you the result. NHS Blood and Transplant says you will find out your blood type after your first donation 5, and in the UK, giving blood is the route the NHS points to 6.
- 2Check your existing recordsIf you have been pregnant, had surgery or had a transfusion, you have almost certainly been typed already. Pregnant women are always given a blood group test 6, so prenatal records and patient portals are a good place to look.
- 3Ask a clinician about a blood typing testA clinician can order ABO and RhD typing, often called a type and screen, when there is a reason for it, such as pregnancy or a planned procedure. It is a standard blood draw 7.
- 4Treat a home kit as a curiosityHome kits test a drop of blood from a fingertip against anti-A, anti-B and anti-D and are read by eye. They skip the back typing a lab uses as a cross-check, and forward-only typing is how the Bombay phenotype gets misread as group O 11.
Why a hospital tests again before a transfusion
Hospitals do not transfuse on the strength of a donor card, an old record or a home kit. Before a planned transfusion, your blood is typed again and tested against a sample of the donor's cells; this crossmatch, followed by a Coombs test, can pick up reactions to minor antigens that routine typing misses 7 6. It is done before transfusions except in emergencies 7, which is when O negative red cells are used. Knowing your type is useful background, and the lab's own test is what decides the blood you receive.
Rh factor and pregnancy
Rh status matters most in pregnancy. When someone who is Rh negative carries a fetus that is Rh positive (inherited from the other parent), it is called Rh incompatibility 8. A pregnant woman and her fetus do not usually share blood, but a small amount of fetal blood can enter the mother's bloodstream, and her body can respond by making anti-D antibodies 8.
How Rh incompatibility can affect a later pregnancy
Anti-D immunoglobulin, called Rh immunoglobulin (RhIg) in the US, is a medicine given as an injection that stops the body from making Rh antibodies if it has not already made them 8. ACOG describes it being offered at about 28 weeks of pregnancy, within 72 hours after the birth of an Rh positive baby, and after events that can mix blood, such as amniocentesis, an ectopic pregnancy, bleeding after 20 weeks or abdominal trauma 8. Each pregnancy needs its own course, and it does not help once antibodies have already formed 8. In the UK, a test on the mother's blood can show the baby's RhD type, and anti-D is offered if the baby is D positive 9. Whether and when you need it is a decision for your obstetric team.
How blood type is inherited
Blood type is inherited, like eye color: you get one ABO gene from each parent 1. A and B are both expressed when present, and O shows up only when a child inherits O from both parents. That is why two group A parents can have a group O child, and why the possible children of each pairing are predictable.
| Parents' blood groups | Possible ABO groups for a child |
|---|---|
| O and O | O |
| O and A | A or O |
| O and B | B or O |
| O and AB | A or B |
| A and A | A or O |
| A and B | A, B, AB or O |
| A and AB | A, B or AB |
| B and B | B or O |
| B and AB | A, B or AB |
| AB and AB | A, B or AB |
Rh follows the same logic. A fetus is Rh negative only when it inherits no RhD gene from either parent 8, so two Rh positive parents can have an Rh negative child, and two Rh negative parents usually have an Rh negative child. The Red Cross notes that testing difficulties and rare variants can break these patterns, and that ABO typing cannot prove or disprove who a parent is 1.
Blood type diet and personality: what the evidence says
The blood type diet claims that eating for your ABO group improves health and lowers disease risk. A systematic review led by researchers at the Belgian Red Cross screened 1,415 references and found no study showing the health effects of ABO blood type diets 12.
No evidence currently exists to validate the purported health benefits of blood type diets.
A later study of 1,455 adults in Toronto tested the idea directly by genotyping each person's ABO group. Some of the diets were linked to better cholesterol, blood pressure and insulin numbers, but the benefit was the same whether or not the diet matched the person's blood type, so the findings do not support the blood type diet hypothesis 13. Any of those eating patterns may help for ordinary reasons, and blood type is not one of them.
Blood type personality theory, widely believed in Japan, fares no better. An analysis of large random-sample surveys of more than 10,000 people in Japan and the US found no meaningful difference between blood groups on 65 of 68 personality items, and blood type explained less than 0.3 percent of the variation in personality 14.
Does blood type affect your health?
Blood type does have some real, modest links to health, and the best established is with blood clots. People with group O have lower levels of two clotting proteins, von Willebrand factor and factor VIII, and a meta-analysis of 38 studies found that people with non-O blood types had about twice the odds of a venous blood clot compared with group O 15. That is a population-level association. It does not tell any one person their risk, which depends far more on things like recent surgery, immobility, pregnancy, hormones and family history, and it is a topic to raise with a clinician, not a reason to change anything on your own.
Putting your blood type in context
Your blood type does not change how the rest of your blood work is read. Ranges for hemoglobin, red cells and the other blood count markers are the same for every type, and you can read those lines with the blood count overview and the guide to how to read any lab result. Blood type earns its place in a few specific moments: a pregnancy, a planned surgery or transfusion, and the decision to donate, where O negative donors and AB plasma donors are always in short supply 1 2.
Link · NHSBlood groupsThe NHS overview of the ABO and Rh systems, how blood group testing works, and why blood group matters in pregnancy and transfusion.nhs.uk
Citations
- American Red Cross. Blood types explained.
- American Red Cross. Know your blood type facts.
- Stanford Blood Center. Blood types: how rare is my type? (general estimates from the AABB Technical Manual, 18th edition).
- Garratty G, Glynn SA, McEntire R; Retrovirus Epidemiology Donor Study. ABO and Rh(D) phenotype frequencies of different racial/ethnic groups in the United States. Transfusion. 2004;44(5):703-706.
- NHS Blood and Transplant. Blood types.
- NHS. Blood groups.
- MedlinePlus Medical Encyclopedia. Blood typing. National Library of Medicine (US).
- American College of Obstetricians and Gynecologists. The Rh factor: how it can affect your pregnancy (patient FAQ).
- NHS. Haemolytic disease of the fetus and newborn (HDFN).
- Shahverdi E, Moghaddam M. Rh(null) phenotype: molecular basis, serologic features, clinical manifestations, and transfusion challenges, a scoping review of published cases. Immunohematology. 2026;42(2):42-47.
- Dipta TF, Hossain AZ. The Bombay blood group: are we out of risk? Mymensingh Med J. 2011;20(3):536-540.
- Cusack L, De Buck E, Compernolle V, Vandekerckhove P. Blood type diets lack supporting evidence: a systematic review. Am J Clin Nutr. 2013;98(1):99-104.
- Wang J, Garcia-Bailo B, Nielsen DE, El-Sohemy A. ABO genotype, 'blood-type' diet and cardiometabolic risk factors. PLoS One. 2014;9(1):e84749.
- Nawata K. No relationship between blood type and personality: evidence from large-scale surveys in Japan and the US. Shinrigaku Kenkyu (Japanese Journal of Psychology). 2014;85(2):148-156.
- Dentali F, Sironi AP, Ageno W, et al. Non-O blood type is the commonest genetic risk factor for VTE: results from a meta-analysis of the literature. Semin Thromb Hemost. 2012;38(5):535-548.
Educational context only, not medical advice or a diagnosis. Always discuss your results with a clinician.
