There are 8 common blood types, created by combining two classification systems: ABO (four groups) and Rh (positive or negative). But the full picture is far more complex. Scientists have identified 48 blood group systems so far, with hundreds of individual antigens on the surface of red blood cells. For everyday medicine, though, those 8 types are what matter most.
The 8 Common Blood Types
Your blood type is determined by two things: which sugar molecules (A and/or B antigens) sit on the surface of your red blood cells, and whether you carry a protein called the RhD antigen. The ABO system gives you one of four groups: A, B, AB, or O. The Rh system then splits each of those into positive (RhD present) or negative (RhD absent). That gives you eight possible combinations: A+, A−, B+, B−, AB+, AB−, O+, and O−.
Each type also comes with built-in antibodies in your plasma that react against the antigens you lack. If you’re type A, your plasma contains anti-B antibodies. Type B plasma contains anti-A. Type O plasma contains both, which is why receiving the wrong blood type during a transfusion can trigger a dangerous immune reaction. Type AB plasma contains neither antibody.
How Common Each Type Is
Blood type distribution varies dramatically by region. Globally, the approximate breakdown looks like this:
- O+: 38.7% (the most common worldwide)
- A+: 27.4%
- B+: 22.0%
- AB+: 5.9%
- O−: 2.6%
- A−: 2.0%
- B−: 1.1%
- AB−: 0.4% (the rarest of the eight)
These averages shift considerably depending on where you look. In the United States, A+ is nearly as common as O+ (35.7% vs. 37.4%), and Rh-negative types are relatively frequent, with O− at 6.6%. In China, O+ dominates at 47.7%, while all Rh-negative types combined account for less than 1% of the population. These regional differences reflect the genetic ancestry of different populations and can create real supply challenges for blood banks.
How Blood Type Is Inherited
The ABO gene comes in three versions, or alleles: A, B, and O. You inherit one from each parent, giving you two copies. A and B are co-dominant, meaning if you inherit one of each, both get expressed and you end up with type AB. The O allele is recessive. You only have type O blood if you inherit the O allele from both parents. Someone with type A blood might carry either two A alleles or one A and one O, which is why two type A parents can sometimes have a type O child.
The Rh gene works independently. It’s inherited on a separate chromosome from ABO, so any ABO group can pair with either Rh-positive or Rh-negative status.
Beyond the Big 8: The Other Blood Group Systems
The International Society of Blood Transfusion currently recognizes 48 blood group systems. The ABO and Rh systems get the most attention because they cause the most severe transfusion reactions, but several others matter clinically. The Kell, Duffy, and Kidd systems, for instance, carry antigens that can cause problems for people who receive multiple transfusions over time. The Kidd system is particularly notorious for causing delayed reactions. The patient’s immune system “remembers” a foreign Kidd antigen from an earlier transfusion and mounts a stronger attack when exposed again, sometimes days or weeks later. These antigens can also cause complications during pregnancy if the mother’s immune system reacts against the baby’s blood cells, though this is typically mild for most minor blood groups.
The 47th system was only discovered recently, when researchers identified the MAL blood group. They found that the AnWj antigen, known for decades but never assigned a home, is carried on a protein called Mal. More than 99.9% of people are AnWj-positive. The rare individuals who lack it carry deletions in the gene that codes for the Mal protein. The 48th system has since been added to the registry, and new ones will likely continue to emerge as genetic sequencing technology improves.
The Rarest Blood Types in the World
Some blood types are so scarce they create genuine medical emergencies when a patient needs a transfusion. Rh-null blood, sometimes called “golden blood,” lacks all Rh antigens, not just the RhD protein. Only about 43 people have ever been reported to have it worldwide. Because Rh-null red blood cells carry none of the Rh markers that could trigger an immune reaction, this blood can theoretically be given to anyone within the Rh system. But the flip side is severe: a person with Rh-null blood can only receive transfusions from another Rh-null donor.
The Bombay phenotype is another exceptionally rare type. People with it lack a foundational molecule called the H antigen, which serves as the building block for A and B antigens. Without it, no A or B antigens can form, so standard blood typing reads them as type O. But they are not type O. Their plasma contains anti-H antibodies in addition to anti-A and anti-B, meaning a transfusion of regular type O blood would trigger a potentially fatal reaction. They can only safely receive red blood cells from other Bombay-type donors. The prevalence varies by population: roughly 1 in 10,000 people in India, 1 in 250,000 among Caucasians, and as rare as 1 in 1,000,000 in Europe.
Why Blood Type Matters for Transfusions
Compatibility rules differ depending on whether you’re receiving red blood cells or plasma, because the immune risks run in opposite directions. For red blood cells, the concern is the antigens on the donated cells. Type O red blood cells carry no A or B antigens, so they won’t trigger an ABO reaction in any recipient. That makes O− the universal red cell donor. Type AB recipients can accept red blood cells from any ABO group, since their plasma lacks both anti-A and anti-B antibodies.
For plasma transfusions, the logic reverses. Now the concern is the antibodies in the donated plasma. Type AB plasma contains no anti-A or anti-B antibodies, making it safe for any recipient. Type O plasma contains both antibodies, so it can only go to other type O patients.
In practice, hospitals try to match your exact type whenever possible. Universal donor blood is reserved for emergencies when there’s no time to test. And for patients with rare types like Bombay or Rh-null, finding compatible blood often requires coordination across national or even international rare blood registries.