The eight blood types are A+, A-, B+, B-, AB+, AB-, O+, and O-. They come from combining two classification systems: the ABO group (A, B, AB, or O) and the Rh factor (positive or negative). Every person has one of these eight types, determined entirely by markers on the surface of their red blood cells.
How the ABO Group Works
Your red blood cells may carry tiny protein markers called antigens. The ABO system is based on two of them, simply named A and B. Which combination you have determines your letter group:
- Type A: Only the A antigen on red blood cells, with anti-B antibodies in the plasma.
- Type B: Only the B antigen on red blood cells, with anti-A antibodies in the plasma.
- Type AB: Both A and B antigens on red blood cells, with no ABO antibodies in the plasma.
- Type O: Neither A nor B antigen on red blood cells, with both anti-A and anti-B antibodies in the plasma.
The antibody part is what makes transfusions tricky. Your immune system naturally produces antibodies against whatever antigen your own cells lack. If type A blood enters a type B person’s body, the recipient’s anti-A antibodies attack those foreign cells. This is why matching blood types matters so much.
What Makes Blood Positive or Negative
The second classification layer is the Rh system, which looks for one specific marker called the RhD antigen. If your red blood cells carry it, you’re positive. If they don’t, you’re negative. That single distinction doubles the four ABO groups into eight total blood types.
Rh status is especially important during pregnancy. When an Rh-negative mother carries an Rh-positive baby, her immune system can recognize the baby’s blood cells as foreign and start producing antibodies against them. This is called Rh incompatibility. To prevent it, Rh-negative mothers typically receive an injection of Rh immune globulin around 28 weeks of pregnancy and again within 72 hours after delivery if the baby turns out to be Rh-positive. The injection stops the mother’s body from forming antibodies that could harm a future pregnancy.
How You Inherit Your Blood Type
Blood type is genetic. You inherit one allele from each parent, and the combination determines your type. There are three possible alleles: A, B, and O. The A and B alleles each code for an enzyme that builds its corresponding antigen on the surface of red blood cells. The O allele codes for a nonfunctional version that produces no surface antigen at all.
A and B are both dominant over O, which means a person with one A allele and one O allele still has type A blood. Someone with one B and one O is type B. But A and B are codominant with each other, so inheriting one of each gives you type AB, with both antigens expressed. The only way to have type O is to inherit the O allele from both parents.
This is why two type A parents can have a type O child. If both carry one A allele and one hidden O allele, there’s a 25% chance they’ll each pass on the O, giving the child type OO. The Rh factor follows a similar pattern: positive is dominant, so two Rh-positive parents who each carry one negative allele can have an Rh-negative child.
Which Blood Types Are Most Common
Blood type distribution varies by population, but in the United States, O-positive is the most common type, and AB-negative is the rarest. Here’s the general ranking from most to least common:
- O+: The most prevalent, found in roughly 37-38% of the US population.
- A+: The second most common, around 30-33%.
- B+: Around 8-9%.
- O-: Around 6-7%.
- A-: Around 6%.
- AB+: Around 3-4%.
- B-: Around 1-2%.
- AB-: Less than 1%, making it the rarest of the eight types.
These percentages shift significantly across ethnic and geographic populations. Type B, for example, is more common in South Asian and East Asian populations, while type O is especially prevalent among Indigenous populations in the Americas.
Transfusion Compatibility
Not every blood type can be safely given to every recipient. The core rule is simple: you cannot receive blood that carries an antigen your body doesn’t recognize, because your immune system will attack it.
O-negative is known as the universal red blood cell donor. Because O-negative cells carry no A antigen, no B antigen, and no RhD antigen, there’s nothing on them for any recipient’s immune system to target. In emergency situations when there’s no time to test a patient’s blood type, O-negative is the go-to choice. This also makes it perpetually in high demand at blood banks despite being relatively uncommon.
AB-positive works in the opposite direction. People with AB+ blood can receive red blood cells from all eight types because their body already carries both A and B antigens plus the RhD antigen. Their immune system won’t react to any of these markers. This makes AB+ the universal recipient for red blood cell transfusions.
For plasma transfusions, the rules reverse. Plasma compatibility depends on antibodies rather than antigens. AB plasma contains no anti-A or anti-B antibodies, making it the universal plasma donor. Type O plasma, which is loaded with both anti-A and anti-B antibodies, can only go to other type O recipients.
Beyond the Eight: Other Blood Group Systems
The eight types most people know about cover the ABO and Rh systems, but they’re far from the whole picture. Scientists have identified 34 additional blood group systems with more than 300 known variants. For routine transfusions, ABO and Rh are what matter most, but these other systems can become relevant for people who need repeated transfusions or in cases of unusual transfusion reactions.
Some of these lesser-known blood groups have fascinating connections to disease. The Duffy blood group, for instance, involves a protein on red blood cells that one species of malaria parasite uses to enter the cell. About 90% of people in sub-Saharan Africa lack this protein entirely, giving them natural resistance to that form of malaria. Another blood group called the S/s system involves a molecule targeted by a different malaria parasite, and some African populations have evolved to lack it as well. These patterns illustrate how blood type diversity isn’t random. It has been shaped over thousands of years by infectious diseases and natural selection.