Your blood type is a label for the specific combination of molecules sitting on the surface of your red blood cells. The most common system uses two markers: a letter (A, B, AB, or O) and a symbol (+ or −). Together, these tell you which proteins and sugars your blood cells carry, which ones they don’t, and what that means for transfusions, pregnancy, and even certain health risks.
What the Letters Mean: The ABO System
The letter in your blood type refers to a type of sugar molecule, called an antigen, attached to the outside of your red blood cells. There are two possible antigens: A and B. Your blood type depends on which ones you have.
- Type A means your red blood cells carry the A antigen.
- Type B means they carry the B antigen.
- Type AB means they carry both.
- Type O means they carry neither.
Here’s the part that makes blood types medically important: your immune system produces antibodies against whichever antigen you don’t have. If you’re type A, your blood contains anti-B antibodies. If you’re type B, you carry anti-A antibodies. Type O blood contains both anti-A and anti-B antibodies. Type AB blood contains no antibodies against either antigen, which is why it’s the most accepting of transfusions from other types.
These antibodies develop naturally within the first few months of life, triggered by bacteria in the gut that resemble blood group antigens. You don’t need a bad transfusion to develop them. Your body builds this defense system on its own.
What the + or − Means: The Rh Factor
The positive or negative sign refers to a separate protein on the surface of red blood cells called the Rh factor (specifically, the D antigen). If you have this protein, you’re Rh positive (+). If you don’t, you’re Rh negative (−). This is inherited, just like your ABO type.
Combining these two systems gives you eight common blood types: A+, A−, B+, B−, AB+, AB−, O+, and O−. Each one describes a unique combination of surface markers on your red blood cells.
How Common Each Blood Type Is
Blood type distribution varies by population, but broadly, O positive is the most common type, and AB negative is the rarest. Data from NHS Blood and Transplant puts the breakdown at:
- O positive: 36%
- A positive: 28%
- O negative: 14%
- A negative: 8%
- B positive: 8%
- B negative: 3%
- AB positive: 2%
- AB negative: 1%
These numbers shift across ethnic and geographic lines. In the United States, about 45% of white Americans are type O, compared to 51% of Black Americans and 57% of Hispanic Americans. Only about 7% of the U.S. population is O negative, which matters because that type is in constant demand for emergencies.
How Blood Type Is Inherited
You inherit one allele of the blood type gene from each parent. The A and B alleles each code for an enzyme that builds a specific antigen on your red blood cells. The O allele codes for a nonfunctional version of that enzyme, so it produces no antigen at all.
A and B are both dominant over O, and they’re co-dominant with each other. That means if you inherit an A from one parent and a B from the other, both antigens show up and you’re type AB. If you inherit an A and an O, the A wins and you’re type A, though you still carry the O allele and can pass it to your children. Two type O parents will always have type O children, because neither parent has an A or B allele to give. But two type A parents could have a type O child if both carry a hidden O allele.
The Rh factor follows a similar pattern. Rh positive is dominant, so you only need one copy of the gene to be Rh positive. Two Rh-negative parents will always have Rh-negative children.
Why Blood Type Matters for Transfusions
If you receive red blood cells carrying an antigen your immune system recognizes as foreign, those antibodies attack the transfused cells. This can trigger a serious, potentially fatal reaction. That’s why matching blood types before a transfusion is critical.
Type O negative is considered the universal red blood cell donor because O negative cells carry no A, B, or Rh antigens for the recipient’s antibodies to target. In emergency rooms, when there’s no time to test a patient’s blood type, O negative red blood cells are the default. Hospitals typically reserve their O negative supply for women of childbearing age, children, and patients known to have certain antibody sensitivities. Men without prior transfusion history may receive O positive in an emergency.
On the receiving end, type AB positive is the universal recipient for red blood cells, since AB+ individuals carry all the major antigens and produce no ABO antibodies. For plasma transfusions, the rules flip: AB plasma is the universal donor because it contains no anti-A or anti-B antibodies that could harm the recipient’s cells.
Blood Type and Pregnancy
The Rh factor carries special significance during pregnancy. If an Rh-negative mother is carrying an Rh-positive baby, her immune system may recognize the baby’s Rh protein as foreign and begin producing antibodies against it. This usually isn’t a problem in a first pregnancy, because the mother’s body hasn’t built up a strong antibody response yet. But in subsequent pregnancies with Rh-positive babies, those antibodies can cross the placenta and attack the baby’s red blood cells, causing a condition called hemolytic disease.
To prevent this, Rh-negative mothers receive an injection that suppresses their immune response to Rh-positive blood cells. This is given around 26 to 28 weeks of pregnancy and again within 72 hours of delivery if the baby turns out to be Rh positive. It’s also given after miscarriage, ectopic pregnancy, amniocentesis, or any event where fetal and maternal blood might mix. This preventive treatment has made severe Rh-related complications rare in countries where it’s routinely available.
Health Risks Linked to Blood Type
Your blood type isn’t just relevant in hospitals. A growing body of research connects ABO type to certain disease risks, particularly for cardiovascular conditions. A large genetic study published in Arteriosclerosis, Thrombosis, and Vascular Biology found that people with non-O blood types (A, B, or AB) face up to 1.5 times the risk of blood clots compared to type O. This includes deep vein thrombosis and pulmonary embolism. The likely explanation is that non-O blood types carry higher levels of a clotting protein called von Willebrand factor.
Type A specifically is associated with modestly higher risks of heart failure (about 14% higher than type O), high cholesterol (9% higher), and hardening of the arteries. Type B carries a 13% increased risk of heart attack compared to type O. These are population-level statistics, not destiny. Your diet, activity level, smoking status, and other factors play a far larger role in your individual cardiovascular risk than your blood type does.
Interestingly, type O isn’t universally protective. People with type O blood have a slightly higher risk of high blood pressure compared to other types. Type O also appears to offer some resistance to severe malaria, which may explain why type O is more prevalent in populations from regions where malaria has historically been common.
Rare Blood Types Beyond ABO and Rh
The eight standard blood types cover most clinical situations, but the full picture is more complex. Scientists have identified over 40 blood group systems with hundreds of antigens. Most of these rarely cause problems, but a few are medically significant.
The Bombay phenotype is one of the rarest. People with this type lack a foundational molecule called the H antigen, which is the building block for both A and B antigens. Their blood tests as type O, but it’s incompatible with normal type O blood. It occurs in roughly 1 in 10,000 people in parts of India and is far rarer elsewhere.
Rh-null, sometimes called “golden blood,” is even more extreme. People with this type lack all Rh antigens, not just the D antigen that determines positive or negative status. Fewer than 50 people worldwide are known to have it. Their red blood cells can be donated to anyone in the Rh system, making these donors extraordinarily valuable, but they themselves can only receive Rh-null blood. Some rare blood types have fewer than 10 registered donors on the planet, which is why international rare donor registries exist to coordinate across borders when a match is needed.