What Blood Type Most Common?

Blood type is one of those things most people know about themselves but rarely think about. If you have ever wondered where you fit in, the answer is straightforward. The most common blood type worldwide is O positive. It is also the most common blood type in the United States.

Roughly 37 to 38 percent of people worldwide have O positive blood, according to data from the American Red Cross and other blood collection organizations. In the US, about 38 percent of the population is O positive. When you combine O positive and O negative, the O group makes up nearly half of all people. That matters for hospitals, blood banks, and anyone who has ever needed a transfusion.

What Blood Type Most Common in the US and Around the World?

O positive leads in both the US and globally. But the full picture is more interesting than a single number.

Blood type distribution shifts depending on where you look. In the US, the order from most to least common is roughly: O positive, A positive, B positive, O negative, A negative, AB positive, B negative, and AB negative. About 38 percent of Americans are O positive and about 34 percent are A positive. The remaining types trail well behind.

Globally, O positive is also the most common, but regional differences are substantial. Parts of South America and Central America have very high rates of O positive blood. Some regions of Asia have higher rates of B positive than you would find in the US or Europe. Indigenous populations in the Americas tend to have very high O frequency. These patterns reflect population history and genetics more than anything else.

AB negative is the rarest type in the US, at less than 1 percent of the population. AB positive is also uncommon, at roughly 3 to 4 percent.

Why Does Blood Type Matter?

Blood type matters most in two situations: blood transfusions and pregnancy.

Your blood type is determined by antigens on the surface of your red blood cells. These are proteins and sugars that your immune system uses to identify your own cells. If someone receives blood with antigens their body does not recognize, their immune system attacks those cells. That reaction can be severe and sometimes life-threatening.

This is why O negative blood is called the universal donor type. O negative red blood cells have neither A nor B antigens and are Rh negative, so they can be given to patients of any blood type in an emergency before the recipient’s type is confirmed. O positive blood can be given to anyone who is Rh positive, which covers most of the population.

People with AB positive blood are called universal recipients for red blood cells. They can receive red cells from any blood type because their immune system already recognizes both A and B antigens and the Rh factor.

In pregnancy, Rh incompatibility is the main concern. If an Rh negative mother carries an Rh positive baby, her immune system can develop antibodies against the baby’s red blood cells. This is preventable with Rh immunoglobulin treatment, which is a well-established part of prenatal care in countries with good medical infrastructure. Without prevention, the condition can cause hemolytic disease of the newborn, which is serious but now rare in settings where Rh immunoglobulin is used.

How Is Blood Type Determined?

Blood type is inherited. You get one allele from each parent, and the combination determines your ABO type and your Rh factor.

The genetics are fairly well understood. The ABO gene has three main versions: A, B, and O. A and B are dominant over O. If you inherit an A from one parent and an O from the other, you have type A blood. If you inherit O from both parents, you have type O. The Rh factor works similarly, with Rh positive being dominant over Rh negative.

This means two parents with type A blood can have a child with type O blood, if both parents carry the O allele. It also means two Rh positive parents can have an Rh negative child, if both carry the Rh negative allele. Blood type does not follow the simple pattern many people assume.

You cannot reliably predict a child’s blood type from the parents’ types alone without knowing the underlying genotype. A lab test is the only way to know for certain.

Does Blood Type Affect Your Health?

Blood type has been studied in relation to many health conditions. The evidence varies widely in quality, and some claims are much better supported than others.

The strongest established link is between blood type and certain risks related to blood clotting. Some research indicates that people with non-O blood types (A, B, and AB) have a somewhat higher risk of venous thromboembolism, which is a blood clot in a vein. The mechanism is reasonably well understood: people with non-O types tend to have higher levels of von Willebrand factor and factor VIII, two proteins involved in clotting. This is a real association supported by multiple studies, though the absolute risk for any individual remains low.

There is also a well-documented association between blood type and susceptibility to certain infections. People with type O blood appear somewhat more susceptible to norovirus and to Vibrio cholerae, the bacterium that causes cholera. People with type A blood may be more susceptible to certain strains of Helicobacter pylori, a bacterium linked to stomach ulcers and stomach cancer. These associations are established in the research literature, though they do not mean that blood type determines whether you get sick.

Other claims are much weaker. The idea that blood type should dictate your diet — the so-called blood type diet — has no solid clinical evidence behind it. A study published in 2014 in the journal PLOS ONE found no association between blood type and the health outcomes the diet claims to improve. No major medical organization endorses eating for your blood type.

Claims linking blood type to personality are not supported by scientific evidence. The idea originated in Japan in the early twentieth century and persists culturally, but controlled studies have not found a reliable relationship.

Can Your Blood Type Change?

Your blood type is set by your genes and does not change under normal circumstances. But there are exceptions worth knowing about.

In some medical situations, a person’s blood type can appear to change on lab tests. Certain cancers, particularly leukemia, can alter the antigens on red blood cells. Bone marrow transplants can also change a person’s blood type, because the new marrow produces blood cells with the donor’s type. In these cases, the change is real but is a consequence of disease or treatment, not a natural shift.

Some infections and rare conditions can cause what looks like a change in blood type on testing, but the underlying genetics have not changed. For practical purposes, though, most people will have the same blood type for life.

How Common Is Each Blood Type?

Here is the approximate distribution of blood types in the US population, based on data from major blood collection organizations.

• O positive: about 38 percent
• A positive: about 34 percent
• B positive: about 9 percent
• O negative: about 7 percent
• A negative: about 6 percent
• AB positive: about 3 percent
• B negative: about 2 percent
• AB negative: about 1 percent

These figures are approximate and vary slightly between sources. They also vary by racial and ethnic group. For example, B positive blood is more common among Black Americans and Asian Americans than among white Americans. O positive is particularly common among Hispanic Americans.

The practical takeaway is simple. If you have O positive blood, you share your type with more people than any other group. If you have AB negative, you are in a small minority.

Why Blood Type Distribution Matters for Blood Donation

Blood banks track these distributions closely because supply needs to match demand. O negative blood is always in high demand because it is the only type that can be given to anyone in an emergency. O positive blood is also in high demand because it can be given to the majority of patients.

People with O negative blood are sometimes called universal donors, and their blood is used constantly in trauma settings. That creates a persistent shortage risk for this type. People with AB positive blood are universal plasma donors, which is a different but equally important role.

Knowing your blood type is useful if you donate blood or if you want to understand your own health information. It is not something you need to worry about day to day. For most people, blood type is simply a fact about their body that matters when it matters.