Type O blood means your red blood cells carry neither A nor B antigens on their surface. This makes it the simplest of the four main blood types in the ABO system, and it’s also the most common. About 37 to 53% of the U.S. population is O positive, while another 1 to 8% is O negative. If you have type O, your immune system produces both anti-A and anti-B antibodies, which is why your blood can’t receive A, B, or AB red cells but can be donated to almost anyone.
How Type O Blood Works
Every person’s red blood cells are coated with sugar molecules that act as identity tags. People with type A blood have one kind of sugar (the A antigen), people with type B have another, and people with type AB have both. Type O is different: the gene responsible for adding those sugars produces a nonfunctional enzyme. Instead of attaching an A or B marker, it leaves the underlying structure, called the H antigen, completely unchanged. Your red blood cells are essentially “blank” in terms of ABO markers.
Because your body never encounters A or B antigens on its own cells, it treats them as foreign. That’s why people with type O blood carry both anti-A and anti-B antibodies in their plasma. If A or B red cells enter your bloodstream through a mismatched transfusion, those antibodies attack immediately. This is also why the rules flip for plasma donations: type O plasma, loaded with both antibodies, can only go to other type O recipients, while type AB plasma (which has no antibodies) is the universal plasma donor.
The Genetics Behind Type O
You inherit two copies of the ABO gene, one from each parent. Three versions of this gene exist: A, B, and O. Both A and B are dominant over O, meaning a single copy of either one is enough to put that antigen on your red blood cells. To end up with type O, you need two O alleles, one from each parent. If you inherit an A from one parent and an O from the other, you’ll be type A. If you get a B and an O, you’ll be type B.
This is why two parents who are both type A or both type B can still have a child with type O. Each parent may be carrying a hidden O allele. When both pass that O allele to the same child, the result is type O blood. Two type O parents, on the other hand, will always have type O children, since neither parent has an A or B allele to pass along.
Why O Negative Is the Emergency Blood Type
Beyond the ABO system, there’s a second major marker called the Rh factor (also known as the D antigen). If you have it, you’re Rh positive; if not, Rh negative. About 82% of Caucasians and an even higher percentage of other racial groups are Rh positive. Rh-negative patients can develop antibodies against Rh-positive blood, so the safest emergency option is blood that lacks every major antigen: O negative.
Hospitals and emergency vehicles stock both O negative and O positive blood for situations where there’s no time to determine a patient’s type. O negative goes first to women of childbearing age to avoid triggering immune reactions that could affect a future pregnancy. O positive, which is far more available, is used for other trauma patients who need blood immediately. Because O-type red blood cells lack A and B antigens, they don’t trigger the anti-A or anti-B antibodies present in any recipient’s plasma. That compatibility is what makes type O the backbone of emergency blood supplies, and why it’s frequently in short supply.
Lower Blood Clot Risk
One of the most consistent health findings linked to type O is a lower risk of blood clots. In a large prospective study, people with non-O blood types (A, B, or AB) had an 86% higher risk of developing a pulmonary embolism with no identifiable cause compared to people with type O. Over a ten-year follow-up, the rate of these clots was roughly 0.11% for type O versus 0.19 to 0.21% for the other types. That gap widened significantly among smokers: non-O smokers had more than 2.5 times the clot risk of type O smokers.
The reason appears to involve clotting proteins. People with A, B, or AB blood tend to have higher circulating levels of a key clotting factor called von Willebrand factor and factor VIII. Type O individuals naturally carry less of these proteins, which may explain their lower tendency to form dangerous clots.
Higher Susceptibility to Stomach Ulcers
The trade-off is that type O blood has long been associated with a higher risk of peptic ulcers. Research has shown that stomach cells from people with blood group O release significantly more inflammatory signaling molecules when exposed to Helicobacter pylori, the bacterium responsible for most stomach ulcers. The bacterium also colonizes the stomach lining more densely in type O individuals. This stronger inflammatory response likely contributes to the increased susceptibility to ulceration that has been observed in type O populations for decades.
Protection Against Severe Malaria
In regions where malaria is endemic, type O blood appears to offer a survival advantage. A study of 567 children in Mali found that group O was present in only 21% of severe malaria cases, compared to 44 to 45% of uncomplicated cases and healthy controls. That translates to a 66% reduction in the odds of developing severe malaria.
The mechanism involves a process called rosetting, where malaria-infected red blood cells clump together with uninfected cells and stick to blood vessel walls. In type O blood, these rosettes are smaller and fall apart more easily than in A, B, or AB blood. Fewer and weaker clumps mean less blockage of small blood vessels and less organ damage. This protective effect is one reason researchers believe type O became so common in tropical populations. It joins a list of red blood cell variations, including sickle cell trait and thalassemia, that appear to have been naturally selected because they reduce malaria severity.
The Blood Type Diet Has No Scientific Support
You may have heard that people with type O blood should eat a high-protein, low-carbohydrate diet. This idea, popularized in the late 1990s, claims that each blood type evolved alongside specific dietary patterns and that eating accordingly improves health. The evidence doesn’t back this up.
A study published in PLOS One tested the hypothesis directly by analyzing diet patterns and health markers across blood types. People who followed the so-called type O diet did see lower triglyceride levels, but here’s the key finding: the benefit was identical regardless of the person’s actual blood type. Someone with type A or type B blood got the same triglyceride improvement from following the “type O diet.” The association had nothing to do with blood type and everything to do with the diet itself being lower in carbohydrates. A systematic review found no study that directly supported matching diets to blood types. The bottom line: a diet that works well for type O works equally well for everyone else.
What Type O Means for Donations
If you’re O positive, your red blood cells are compatible with all Rh-positive recipients: A+, B+, AB+, and O+. That covers the vast majority of the population. If you’re O negative, you can donate to anyone, making your blood the most versatile and most needed type in hospitals. O negative makes up only about 7% of the population, yet it’s the first choice in every emergency where the patient’s blood type is unknown.
For receiving blood, the situation is more restrictive. Because your plasma contains both anti-A and anti-B antibodies, you can only receive red blood cells from other type O donors. O positive recipients can take O positive or O negative blood. O negative recipients can only receive O negative. For plasma transfusions, type O individuals can receive plasma from any ABO type, since it’s the antibodies in the plasma (not the antigens) that matter, and your body already has both.