There are eight main blood types, determined by two systems: ABO (which sorts you into A, B, AB, or O) and Rh factor (positive or negative). Combined, these produce A+, A−, B+, B−, AB+, AB−, O+, and O−. Your blood type is set at birth, inherited from your parents, and never changes.
The ABO System
Your blood type depends on which sugar molecules, called antigens, sit on the surface of your red blood cells. Type A blood carries the A antigen. Type B carries the B antigen. Type AB carries both. Type O carries neither.
What makes this matter is the flip side: your plasma (the liquid part of blood) contains antibodies that attack whichever antigens your own cells lack. If you’re type A, your plasma carries anti-B antibodies. If you’re type B, you carry anti-A antibodies. Type O blood has both anti-A and anti-B antibodies. Type AB has neither, which is why AB individuals can receive red blood cells from any ABO group.
The Rh Factor
The second piece of your blood type is the Rh factor, specifically a protein called RhD. If your red blood cells have it, you’re Rh positive (+). If they don’t, you’re Rh negative (−). About 85% of people worldwide are Rh positive.
Rh status is especially important during pregnancy. When an Rh-negative mother carries an Rh-positive baby, small amounts of the baby’s blood can enter the mother’s bloodstream, typically during delivery. Her immune system may recognize the RhD protein as foreign and build antibodies against it. This usually doesn’t cause problems in a first pregnancy, but in a later pregnancy with another Rh-positive baby, those antibodies can cross the placenta and attack the baby’s red blood cells, causing a serious form of anemia. A preventive injection given during pregnancy and after delivery stops the mother’s body from forming these antibodies in the first place.
How Common Each Type Is
In the United States, the eight blood types break down roughly like this:
- O positive: 38%, the most common
- A positive: about 34% (roughly 1 in 3 people)
- B positive: about 9%
- O negative: 7%
- A negative: about 6% (1 in 16)
- AB positive: less than 4%
- B negative: less than 2%
- AB negative: less than 1%, the rarest among Americans
These numbers shift considerably around the world. In Ecuador and Peru, type O positive accounts for 70 to 75% of the population, while type B is relatively scarce. In India and Pakistan, type B positive is the most common group, found in roughly 34 to 38% of people. Japan has unusually high rates of type A positive (about 40%) and a more even spread across ABO groups. Globally, the averages sit at about 39% O+, 27% A+, 22% B+, and 6% AB+, with Rh-negative types collectively making up only about 6% of the world’s population.
Who Can Donate to Whom
Blood transfusion compatibility follows a straightforward logic: you can’t receive blood that carries antigens your body would attack. Type O negative red blood cells have no A, B, or RhD antigens on their surface, so virtually anyone can receive them safely. That’s why O negative is called the universal red cell donor, and it’s the type hospitals reach for in emergencies when there’s no time to test a patient’s blood.
Type O can donate red cells to anybody but can only receive from other type O donors. Type A can donate to A and AB recipients. Type B can donate to B and AB. Type AB can donate red cells only to other AB individuals but can receive from all groups, making AB positive the universal red cell recipient.
Plasma donation works in the opposite direction. Because AB plasma contains no anti-A or anti-B antibodies, it’s safe for all recipients regardless of their blood type. That makes AB the universal plasma donor. Type O plasma, loaded with both types of antibodies, can only go to other O recipients.
How Blood Type Is Inherited
You inherit one ABO gene from each parent, giving you two copies. The A and B versions are co-dominant, meaning if you get one of each, both are expressed and your blood type is AB. The O version is recessive, so it only shows up when you inherit it from both parents.
This means someone with type A blood could carry either two A copies (AA) or one A and one O (AO). The same applies to type B: either BB or BO. Type AB always comes from one A gene and one B gene. Type O requires two O copies (OO). Because of these hidden carrier combinations, two parents who are both type A can have a child with type O if each parent passes along their silent O gene.
Rh factor follows a simpler pattern. The gene for RhD positive is dominant. You only need one copy to be Rh positive. Two Rh-positive parents can have an Rh-negative child if both carry one recessive copy.
Extremely Rare Blood Types
Beyond the familiar eight, there are hundreds of additional blood group systems recognized by the International Society of Blood Transfusion. Most people never need to think about them, but a small number of individuals carry blood types so unusual that finding a compatible donor becomes a global challenge.
The Bombay phenotype (also called Oh) is one well-known example. People with this type lack a foundational molecule that the A and B antigens are built on, so standard blood typing can misidentify them as type O. They can only receive blood from other Bombay-type donors, an extremely small group. Rh-null, sometimes called “golden blood,” is even rarer. People with Rh-null lack all Rh antigens, not just RhD. Fewer than 50 people worldwide are known to have it. Their red cells can theoretically be given to anyone in the Rh system, but finding replacement blood for them when they need a transfusion is extraordinarily difficult.
The ISBT defines rare blood as being negative for a high-prevalence antigen found in fewer than 1 in 1,000 people. Some types are so scarce that fewer than 10 registered donors exist in the entire world, which is why international rare donor registries coordinate across countries to locate compatible blood when it’s needed.
Finding Out Your Blood Type
If you’ve donated blood, had surgery, or been pregnant, your type is likely already on file. You can ask your doctor’s office or blood bank for the result. Blood donation centers like the Red Cross will also tell you your type after your first donation. At-home blood typing kits are available at pharmacies and work by mixing a drop of your blood with solutions that cause clumping if certain antigens are present. They’re generally reliable for ABO and Rh typing, though a lab test is more definitive.
Knowing your blood type won’t change your daily life, but it’s useful information in emergencies, during pregnancy planning, and if you want to donate blood. Hospitals always verify blood type before a transfusion regardless of what you report, so even if you’re unsure, you’ll be tested when it counts.