There are 8 common blood types based on the two classification systems used in everyday medicine: A+, A-, B+, B-, AB+, AB-, O+, and O-. But the full picture is much larger. Scientists have identified 44 distinct blood group systems so far, with the most recent discovered in 2022. The total number of recognized blood group antigens (the surface markers that define types) runs into the hundreds.
For most people, the 8 types formed by combining the ABO and Rh systems are all that matter for transfusions and pregnancy. Here’s what each system means and why the rarer ones occasionally come into play.
The ABO System
Your ABO type depends on which sugar molecules sit on the surface of your red blood cells. Type A cells carry one kind, type B cells carry another, type AB cells carry both, and type O cells carry neither. Your immune system produces antibodies against whichever markers your own cells lack, which is why receiving the wrong blood type triggers a dangerous reaction.
Type O is the most common group worldwide. About 57% of Hispanics, 51% of African Americans, and 45% of Caucasians are type O. Type AB is the rarest of the four, found in roughly 3% of donors.
The Rh System
The second major classification is the Rh factor, a protein on the surface of red blood cells. If you have it, you’re Rh-positive; if you don’t, you’re Rh-negative. Combining ABO with Rh gives the familiar 8 types. Based on UK donor data, their approximate frequencies are:
- O+: 36%
- O-: 14%
- A+: 28%
- A-: 8%
- B+: 8%
- B-: 3%
- AB+: 2%
- AB-: 1%
These percentages shift depending on ethnicity and geography. Type B, for example, is more common in Asian populations, while type O dominates in Indigenous and Hispanic populations.
Why Rh Matters in Pregnancy
Rh factor has a unique significance beyond transfusions. If you’re Rh-negative and your baby is Rh-positive (inheriting the factor from the other parent), your immune system can treat the baby’s blood cells as foreign and build antibodies against them. This rarely causes problems in a first pregnancy, but those antibodies can attack a future Rh-positive baby’s red blood cells, leading to severe anemia.
The standard prevention is an injection given around 26 to 28 weeks of pregnancy and again within 72 hours of delivery. This treatment stops the immune system from producing those antibodies in the first place, and it has made serious Rh complications far less common than they once were.
The 44 Blood Group Systems
ABO and Rh get all the attention, but they’re just 2 of 44 recognized blood group systems. Each system is defined by a different set of proteins or sugars on the red blood cell surface. Most of them rarely cause problems, but a few can matter clinically.
The Kell system is the third most significant after ABO and Rh. The Kell antigen appears in about 9% of Caucasians, 2% of Black individuals, and is rare in Asian populations. Antibodies against Kell antigens can cause severe fetal anemia during pregnancy, similar to Rh incompatibility. People who receive frequent transfusions, such as those with sickle cell disease, are sometimes matched for Kell in addition to ABO and Rh to prevent reactions.
The newest system, called Er, was identified in 2022 by researchers at the University of Bristol. They used DNA sequencing and gene-editing techniques to trace it to a protein called Piezo1 on the red blood cell surface. Piezo1 plays a role in many biological processes throughout the body, but specific genetic changes alter its structure enough to create a distinct blood group. Er became the 44th system to be officially recognized.
Exceptionally Rare Types
Some blood types are so uncommon they create real logistical challenges for transfusion services.
The Bombay phenotype looks like type O on routine testing but is fundamentally different. All ABO types are built on a foundation molecule called the H antigen. Type A and B cells convert most of their H antigens into A or B markers, while type O cells leave the H antigen unconverted. People with the Bombay phenotype lack the H antigen entirely, so they can’t produce A, B, or even the unconverted H found in type O. Their immune system treats H as foreign, meaning they react to transfusions from every standard blood type, including O. They can only receive blood from other people with the Bombay phenotype.
Then there’s Rh-null, sometimes called “golden blood.” People with this type lack all Rh antigens on their red blood cells, not just the main Rh-positive/negative marker, but every protein in the entire Rh system. Only about 43 people have ever been reported to have it, according to Cleveland Clinic. Rh-null blood can theoretically be donated to anyone who is Rh-negative, making it extremely valuable. But with so few known carriers spread across the globe, finding a compatible donor in an emergency is nearly impossible.
What “Universal Donor” Actually Means
O-negative is often called the universal donor type because O-negative red blood cells lack A, B, and Rh markers, meaning they’re unlikely to trigger an immune reaction in any recipient. Hospitals use O-negative blood in emergencies when there’s no time to test a patient’s type.
This label only applies to red blood cell transfusions, though. Blood products work differently depending on what’s being transfused. For plasma donations, the compatibility rules essentially reverse: AB plasma is considered universally compatible because it contains no antibodies against A or B antigens.
Only about 14% of donors are O-negative and just 3% are AB, which is why blood banks are constantly in need of both. If you know your blood type is uncommon, donating regularly can make an outsized difference.
How Blood Types Are Determined
Your blood type is inherited, determined by genes from both parents. The A and B genes are co-dominant (which is why AB exists), while O is recessive. You could carry a hidden O gene and still test as type A or B. Rh follows a similar pattern: Rh-positive is dominant, so two Rh-positive parents can have an Rh-negative child if both carry the recessive gene.
Typing is done by mixing a small blood sample with antibodies that react to specific antigens. If the blood clumps when exposed to anti-A antibodies, you’re type A. If it clumps with anti-B, you’re type B. Clumping with both means AB, and no clumping means O. A separate test with anti-Rh antibodies determines positive or negative status. The whole process takes minutes in a lab, and most people learn their type through a blood donation, a pregnancy screening, or a routine preoperative test.