The rarest blood type is Rhnull. Unlike other blood types, people with Rhnull blood have no antigens on their red blood cells.
Researchers estimate that just 1 in 6 million people have Rhnull blood.
Healthcare professionals classify blood type according to the presence or absence of antigens, which are proteins attached to red blood cells.
In this article, learn more about the rarest blood type. We also cover other blood types, including whether or not they are compatible with each other.
The American Red Cross define a blood type as “rare” when it occurs in fewer than 1 in 1,000 people. Rhnull is the rarest of these.
Having a rare blood type can make it difficult or even impossible to get a blood transfusion or organ transplant.
It can also cause other health issues. For example, if their blood is incompatible with a developing fetus, pregnant women with rare blood types may experience complications. One Iranian woman with Rhnull blood had several pregnancy losses.
Most blood types fall into one of four blood type groups, according to whether they contain A or B antigens.
For example, people with A antigens have type A blood, while those with B antigens have type B blood. People with both A and B antigens have type AB blood, while people with neither antigen on their red blood cells have type O blood.
In addition to the blood group type, a person may also carry Rh factor on their red blood cells. A person without Rh factor has Rh- blood, while someone with it has Rh+ blood. For example, a person with AB blood and Rh factor has AB+ blood.
Some people, including those with Rhnull blood, lack one or more common antigens. There are more than 30 other known blood groups and more than 600 antigens.
Although most people have blood that falls into one of four blood group types, these types vary in prevalence across ethnic groups and geographic regions.
According to data from the Stanford School of Medicine Blood Center, AB- blood is the rarest type in the United States. Just 0.6% of people in the U.S. have this blood type.
The prevalence of the other common blood types in the U.S. is as follows:
- O+: 37.4%
- O-: 6.6%
- A+: 35.7%
- A-: 6.3%
- B+: 8.5%
- B-: 1.5%
- AB+: 3.4%
- AB-: 0.6%
Blood type prevalence varies across populations and geographic regions.
Researchers once sought to classify people into distinct races based upon blood groups, but blood types do not fit into racial categories.
Across geographic regions, O blood groups are the most common. In fact, around 63% of the world’s population has this blood group.
In Central and South America, the rate of O blood is much higher — close to 100% in some regions. In Eastern Europe and Central Asia, the rate of O blood is lower, and B blood is more common.
That being said, B blood is the least common group across the globe. Only around 16% of the world’s population has it.
Across the world, Rh+ blood is much more common than Rh- blood. The lowest known rate of Rh+ blood is still high, at around 65% among the Basque people of the Pyrenees mountains. Sub-Saharan African populations have the highest rate of Rh+ blood, at around 97–99%.
Blood type is a genetic trait. However, a child can have a different blood type to both of their parents, depending on which genes they inherit.
Each parent passes one allele (gene) for blood type to their child. A parent who has O blood can only pass an O allele.
A and B alleles are “co-dominant.” This means that a child who inherits one of each will have AB blood.
For example, a mother with type A blood who passes on an A allele and a father with type B blood who passes on the B allele will have a child with AB blood.
Parents pass on Rh factor in the same way. Rh+ blood is dominant. This means that if a child inherits one Rh+ allele and one Rh- allele, the child will have Rh+ blood. To be Rh-, the child must inherit two Rh- alleles. This is because it is recessive.
In 2015, researchers identified an enzyme that could “cut” antigens from blood cells. In theory, this would allow doctors to change a person’s blood type, potentially making it easier for them to get a transfusion.
However, the researchers had to use very large quantities of the enzyme, and they did not test their theory in human participants. Although it might one day be possible, doctors cannot currently change a person’s blood type.
A person can only give blood to someone with compatible blood antigens.
People with Rh- blood can give blood to both Rh- and Rh+ recipients. However, those with Rh+ blood cannot give to Rh- recipients.
If a person receives blood from someone with an incompatible blood type, it can cause a life threatening immune system reaction. The blood transfusion will likely fail.
A person with type O blood can donate to anyone, as long as the Rh factor is compatible. This means that people with O+ blood can donate to someone with A+, AB+, B+, or O+ blood, but not to people with O-, B-, AB-, or A- blood.
O- is a universal donor, which means that a person with this blood type can donate to anyone.
A person with type AB blood is a universal recipient, as long as the Rh factor is compatible. This means that they can receive blood from all other blood groups.
People with group AB, A, or B blood can only donate to people with the same blood group type.
Having a rare blood type makes it more difficult for a person to receive a blood transfusion. It can also increase the risk of certain health complications, especially following an organ transplant and during pregnancy.
People who are curious about their blood type can ask a doctor for a quick blood test. Many people learn their blood type when they donate blood for the first time.
People with rare blood types should ask a doctor about access to safe blood transfusions.