O positive (O+) is the most common blood type, carried by about 37% of the U.S. population. It means your red blood cells lack both A and B surface markers, and they do carry a protein called the Rh factor. This combination makes O+ blood especially useful in transfusions and gives it a distinct genetic and health profile worth understanding.
What Makes Blood “O Positive”
Your blood type is determined by two separate systems working together. The first is the ABO system, which looks at sugar molecules (antigens) on the surface of your red blood cells. Type A blood has A antigens, type B has B antigens, type AB has both, and type O has neither. If you’re type O, your red blood cells are essentially “blank” when it comes to these markers.
The second system is the Rh system, which checks for a specific protein called the RhD antigen. If your cells carry this protein, you’re Rh positive (+). If they don’t, you’re Rh negative (-). So O+ means: no A antigens, no B antigens, and the RhD protein is present.
How O+ Is Inherited
You inherit two copies of the ABO gene, one from each parent. The gene comes in three versions: A, B, and O. Both A and B are dominant over O, which means a single copy of A or B will determine your blood type. To end up with type O, you need two copies of the O version, one from each parent. A parent with type A blood, for example, might carry a hidden O gene (AO) and pass that O along to you.
The Rh factor works the same way. The positive version is dominant, so you only need one copy to be Rh positive. Someone who is O+ might carry two positive copies (+/+) or one positive and one negative (+/-). If both parents carry that hidden negative copy, they could have an O negative child even though both parents are O positive.
Who Can Receive O+ Blood
Because O+ red blood cells have no A or B antigens, they won’t trigger an immune reaction in anyone who is Rh positive. That means O+ red blood cells can go to anyone with A+, B+, AB+, or O+ blood. This covers a large majority of the population and makes O+ donors extremely valuable to blood banks.
The one limitation is the Rh factor. People who are Rh negative can develop an immune response to Rh positive blood, so O+ cannot safely go to O-, A-, B-, or AB- recipients in most situations. That distinction is why O negative, not O positive, is considered the true universal donor for red blood cells.
If you’re the one receiving blood, your options are more limited. O+ recipients can only receive red blood cells from O+ or O- donors, since any A or B antigens from other types would cause a reaction.
O+ Blood in Emergencies
When someone arrives at a hospital bleeding heavily and there’s no time to test their blood type, doctors reach for type O. The standard protocol reserves O negative blood for women of childbearing age, because Rh incompatibility could complicate a future pregnancy. For everyone else, including men and postmenopausal women, O positive is the preferred emergency blood. The risk of a serious transfusion reaction from giving Rh positive blood to an Rh negative person in an emergency is less than 1%, and reactions that do occur are typically mild.
This is one reason blood banks are constantly in need of O+ donations. It serves the broadest range of emergency patients while also being the most commonly needed type for routine transfusions.
O+ Plasma Works Differently
Blood type compatibility flips when it comes to plasma. Your plasma contains antibodies against the antigens your red blood cells don’t have. Since type O blood lacks both A and B antigens, O plasma contains antibodies against both. That means O plasma can only go to other type O recipients. If you’re O+ and want to help the most people, donating whole blood or red blood cells is more impactful than donating plasma.
Heart Disease and Blood Clot Risk
Type O blood is associated with the lowest risk of coronary artery disease among all blood types. People with type A face about 5% higher risk, type B about 10% higher, and type AB about 23% higher compared to type O. The difference is thought to relate to clotting proteins that circulate at lower levels in people with type O blood.
This doesn’t mean O+ is a free pass. Lifestyle factors like diet, exercise, and smoking still have a far larger influence on heart disease risk than blood type. But it’s a small, built-in advantage.
Protection Against Severe Malaria
Type O blood offers measurable protection against severe malaria, and researchers at Karolinska Institutet identified a key reason why. The malaria parasite produces a protein called RIFIN that travels to the surface of infected red blood cells and acts like glue, causing cells to clump together. This clumping is what makes malaria dangerous, blocking small blood vessels and damaging organs. RIFIN bonds strongly to type A blood cells but only weakly to type O, which means infected O-type cells are less likely to form the dangerous clumps that lead to severe illness. This is one reason type O blood is more common in regions where malaria has historically been prevalent, with 51% of African Americans and 57% of Hispanic Americans carrying some form of type O.
The Blood Type Diet Is Not Supported by Evidence
You may have heard that people with type O blood should eat a high-protein, meat-heavy diet. This idea comes from the “blood type diet” popularized in the late 1990s, but it has no scientific backing. A 2013 review of the world’s medical literature found zero studies demonstrating any benefit from eating based on blood type. A follow-up study in 2014 found that people who followed blood-type diets did see some improvements in cholesterol and blood pressure, but those improvements had nothing to do with their actual blood type. People of any type who followed the same diet got the same results.
There is also no established connection between blood type and digestion. A 2021 study of people on a low-fat vegan diet found no link between blood type and lipid levels or other metabolic markers. If a blood-type diet happens to improve your health, it’s because the diet itself is reasonable, not because it matches your blood.