Blood type O means your red blood cells carry neither A nor B antigens on their surface. These antigens are sugar molecules that sit on the outside of red blood cells, and they’re what the immune system uses to tell “self” from “foreign.” Because type O cells lack both markers, your plasma contains antibodies against both A and B blood types, ready to attack those cells if they enter your bloodstream. About half of people have some form of type O blood, making it the most common blood group worldwide.
How Type O Blood Works
Your blood type is determined by what’s coating the surface of your red blood cells. People with type A have A antigens, type B have B antigens, and type AB have both. Type O has neither. Think of it as a blank surface. Your immune system, never having seen A or B antigens on its own cells, treats them as invaders. That’s why your plasma carries anti-A and anti-B antibodies at all times.
This matters most during transfusions. If you have type O blood and receive type A, B, or AB red blood cells, your antibodies will attack them, potentially causing a dangerous reaction. You can only receive red blood cells from other type O donors.
O Positive vs. O Negative
The “positive” or “negative” label refers to a separate protein called the Rh factor (or RhD protein) on your red blood cells. O positive means you have the Rh protein; O negative means you don’t. This distinction has real consequences.
O negative blood has no antigens of any kind, which means it won’t trigger an immune response in any recipient regardless of their blood type. This is why O negative is called the “universal donor” type. Hospitals stock it specifically for emergencies when there’s no time to test a patient’s blood type, such as major trauma or massive bleeding. O positive red blood cells, while not universally compatible, can be given to anyone with a positive blood type (A+, B+, O+, AB+), which covers the majority of the population.
Among blood donors in the UK, about 36% are O positive and 14% are O negative. O negative is always in high demand because of its emergency use, and blood banks frequently issue calls for O negative donors.
How You Inherit Type O
Blood type is genetic, and type O follows a specific inheritance pattern. The genes for A and B blood groups are co-dominant, meaning they express equally when present. But the O gene is recessive. To end up with type O blood, you need to inherit an O gene from both of your biological parents.
Each parent carries two copies of the ABO gene. A parent with type A blood might carry one A gene and one O gene. If both parents pass along their O gene, the child will be type O. Two parents who are both type O will always have type O children, since neither parent has an A or B gene to pass on. But two parents with type A or type B blood can also have a type O child if both happen to carry a hidden O gene.
Why Type O Exists
Type O likely evolved because it offered a survival advantage against infectious diseases, particularly malaria. Researchers at Stanford Blood Center note that type O appears to have evolved before humans migrated out of Africa and is especially common in regions where malaria has historically been widespread.
The evidence for this is striking. A study published in the Proceedings of the National Academy of Sciences found that blood group O was associated with a 66% reduction in the odds of developing severe malaria compared with non-O blood groups. Among patients with severe malaria, only 21% had type O blood, compared with 44 to 45% of healthy controls and those with uncomplicated infections. The protective mechanism involves how malaria parasites clump infected red blood cells together. Type O cells resist this clumping more effectively, limiting the damage the parasite can do.
This protection likely came with a trade-off. Type O is thought to increase susceptibility to cholera and other severe diarrheal diseases, which may explain why O didn’t simply replace all other blood types over thousands of generations. Different blood types persisted because each offered advantages in different disease environments.
Heart Disease and Clotting Risk
Type O blood is consistently linked to a lower risk of cardiovascular problems. Harvard Health Publishing reports that type O is associated with the lowest risk of coronary artery disease. Compared to type O, people with type A have a 5% higher risk, type B 10% higher, and type AB 23% higher.
The reason appears to be related to clotting. People with type O blood tend to have lower levels of a clotting protein called von Willebrand factor, which makes their blood slightly less prone to forming the clots that can lead to heart attacks and strokes. This is genuinely protective for cardiovascular health, though it also means type O individuals may bleed a bit more heavily after injuries or surgeries.
What the “Blood Type Diet” Gets Wrong
You may have heard claims that people with type O blood should eat a high-protein, meat-heavy diet to match their “hunter” ancestry. This idea, popularized in the late 1990s, has no scientific support. A systematic review published in The American Journal of Clinical Nutrition searched established medical and scientific databases and found zero studies validating the health benefits of blood type diets. None.
The review’s conclusion was blunt: the widespread claims about blood type diets are theoretical and not supported by evidence. Healthy eating matters, but your blood type doesn’t determine which foods are good for you.
Type O in Emergency Medicine
If you’re ever in a serious accident and need blood before doctors can run a blood test, you’ll almost certainly receive type O red blood cells. The Association for the Advancement of Blood and Biotherapies recommends group O for any patient whose blood type is unknown during emergency resuscitation.
Specifically, women of childbearing age receive O negative blood in these situations to avoid complications with the Rh factor that could affect future pregnancies. Men and postmenopausal women typically receive O positive, since it’s more widely available and the Rh factor poses less risk for them. Once a patient’s actual blood type is determined through testing, they’re switched to type-specific blood as quickly as possible.
This emergency protocol is one reason O negative blood is perpetually in short supply. Only about 14% of donors have it, but it’s the default for the most critical moments in trauma care.