If you have ever donated blood or had a type and screen, you likely know your blood type. The most common blood type in the United States and globally is O. Specifically, O positive is the most frequent Rh-positive type, found in roughly 37% of Americans. O negative, the universal donor type, is much rarer at about 7% of the population. But blood type distributions vary significantly by ancestry and geographic region, making the answer more nuanced than a single number.
What determines your blood type?
Your blood type is inherited from your parents. It is determined by the presence or absence of two key markers on the surface of your red blood cells: the A and B antigens. The ABO blood group system sorts blood into four main types: A, B, AB, and O. A second important marker is the Rh factor (also called the D antigen), which makes a blood type either positive or negative.
If your cells have A antigens but not B, your type is A. If they have B but not A, your type is B. If they have both A and B, your type is AB. If they have neither A nor B, your type is O. The Rh factor works similarly — if you have the D antigen, you are Rh positive; if you lack it, you are Rh negative. So a person could be O positive (most common), O negative, A positive, A negative, B positive, B negative, AB positive, or AB negative.
What is the most common blood type?
In the United States, the most common blood type is O positive. According to data from the American Red Cross, about 37% of the U.S. population is O positive. The next most common types are A positive (around 28%), B positive (about 11%), and O negative (about 7%). AB negative is the rarest at roughly 1%.
Globally, O positive is also the most common, but the exact percentage varies by region. In some parts of Central and South America, O blood types are even more prevalent — exceeding 60% in some Indigenous populations. In Europe, the distribution is more balanced between O and A. Blood type B is more common in Asian populations, while AB remains consistently rare worldwide.
Why does blood type matter?
Blood type matters most in transfusions and pregnancy. A mismatch between donor and recipient blood types can cause a life-threatening immune reaction. People with type O negative blood are considered universal donors because their red blood cells lack A, B, and Rh antigens — so they can be given to almost anyone in an emergency. Conversely, people with AB positive blood are universal recipients for red blood cells because their immune system accepts all ABO and Rh types.
Blood type also plays a role during pregnancy. If an Rh-negative mother carries an Rh-positive baby, her immune system may develop antibodies against the Rh factor. This can lead to hemolytic disease of the newborn in subsequent pregnancies. Routine RhoGAM shots given at key points during pregnancy prevent this problem.
Some studies have suggested links between blood type and risk for certain diseases, such as blood clots, stomach cancer, or memory decline. But these associations are modest and not strong enough to guide medical decisions. No one treatment or screening is recommended based solely on blood type.
How is blood type inherited?
Blood type follows simple Mendelian genetics. You inherit one gene from each parent. The A and B alleles are co-dominant (both expressed if present), and the O allele is recessive. So a person with one A and one O gene becomes type A. A person with two O genes becomes type O. The Rh factor is inherited separately — Rh positive is dominant over Rh negative.
This means two parents who are both type O can only produce children with type O. Two parents with type A or B can produce children with various combinations if they carry an O allele. Blood type inheritance can be predicted with a simple Punnett square, but actual typing is done by a lab test.
Can your blood type change?
Your blood type is fixed for life — it does not change. However, there are rare exceptions. A bone marrow transplant from a donor of a different blood type can permanently change the recipient’s blood type, because the new marrow produces red cells with the donor’s antigens. In some illnesses, such as certain cancers, temporary loss or weakening of A or B antigens can occur. But for virtually everyone, your blood type at birth is your blood type for life.
Some myths claim that diet or stress can change your blood type. There is no scientific evidence to support these ideas.
Frequently Asked Questions
What is the rarest blood type?
In the United States, AB negative is the rarest, found in about 1% of the population. Globally, any blood type can be rare depending on the region, but the Rh-null type is extremely rare — fewer than 50 people worldwide are known to have it.
Is O negative blood always needed?
Yes, O negative blood is the universal red blood cell type used in trauma emergencies when there is no time to test the patient’s blood type. It is also routinely used for newborn transfusions because it lacks all major antigens.
Can two parents with type A have a type O child?
Yes, if both parents carry one A allele and one O allele (genotype AO), they can each pass on the O allele, resulting in an O child. The likelihood is 1 in 4 for each pregnancy.
Does blood type affect diet or exercise?
The blood type diet popularized by some books has not been supported by well-designed scientific studies. There is no reliable evidence that your ABO type dictates what foods you should eat or how you should exercise.
