Type O blood is the most common blood type in the world, and it’s defined by what it lacks: the red blood cells carry neither A nor B antigens on their surface. This absence makes type O unique in the ABO blood group system, especially when it comes to donations and transfusions. About 50% of blood donors have some form of type O, split between O positive (36%) and O negative (14%).
How Type O Blood Forms
All ABO blood types start from the same raw material, a sugar structure called H substance, which sits on the surface of every red blood cell. In people with type A blood, an enzyme adds one specific sugar molecule to that base structure. In type B, a different enzyme adds a different sugar. Type O blood is what happens when neither enzyme is functional. The H substance stays unmodified, with no additional sugar attached. There is no “O antigen” per se. Type O simply means the base structure was left as-is.
This comes down to genetics. The ABO gene on chromosome 9 has three possible versions (alleles): A, B, and O. Both A and B are co-dominant, meaning if you inherit one of each, both express themselves and you end up with type AB. The O allele is recessive. It carries a deletion that prevents the enzyme from being produced at all. To have type O blood, you need to inherit the O allele from both parents, giving you the genotype OO. If even one parent passes along an A or B allele, that allele takes over.
Why O Negative Is the Universal Donor
When someone receives a blood transfusion with mismatched antigens, their immune system treats those foreign blood cells as invaders. Antibodies attack and destroy the transfused cells, triggering a severe reaction that can cause kidney failure and dangerously low blood pressure within minutes.
O negative red blood cells have no A antigens, no B antigens, and no Rh D antigen. There’s nothing on their surface for a recipient’s immune system to recognize and attack. This is why O negative blood is considered universally compatible for red blood cell transfusions and why air ambulances carry it on board for emergencies when there’s no time to test a patient’s blood type. Despite only about 8% of the population being O negative, it accounts for roughly 13% of hospital red blood cell requests.
O positive blood is nearly as versatile. It can go to anyone who is Rh-positive (the majority of people), though not to Rh-negative recipients. When it comes to receiving blood, the rules flip: people with type O can only receive type O red blood cells. O positive recipients can take O positive or O negative. O negative recipients can only receive O negative.
Plasma Works in Reverse
Here’s where things get counterintuitive. Type O blood contains both anti-A and anti-B antibodies in the plasma (the liquid part of blood). That means type O plasma can attack A, B, or AB red blood cells if given to the wrong recipient. So while type O is the best red blood cell donor, it’s actually the worst plasma donor. Type AB plasma, which has no anti-A or anti-B antibodies, is the universal plasma donor. If you have type O blood and ever need a plasma transfusion, you can receive plasma from any blood type. But your plasma can only safely go to other type O recipients.
Cardiovascular and Clotting Risks
Blood type influences more than transfusion compatibility. A large body of evidence links ABO type to cardiovascular health, and type O generally comes out ahead. Compared to people with type O, those with type A or B blood have an 8% higher risk of heart attack and a 10% increased risk of heart failure. The difference in blood clot risk is even more striking: people with type A or B blood are 51% more likely to develop deep vein thrombosis and 47% more likely to develop a pulmonary embolism.
The one area where type O doesn’t have an advantage is blood pressure. People with type A or B actually have a 3% lower risk of high blood pressure compared to type O. But on balance, the overall cardiovascular picture favors type O.
Protection Against Malaria
In regions where malaria is endemic, type O blood offers a measurable survival advantage. The malaria parasite (the species responsible for the most severe form of the disease) infects red blood cells and then secretes a protein called RIFIN, which travels to the surface of the infected cell. This protein acts like glue, causing infected cells to stick together and clog small blood vessels. That blockage cuts off oxygen to tissues and can lead to coma, brain damage, and death.
Research from Karolinska Institutet showed that RIFIN bonds strongly to type A blood cells but only weakly to type O cells. This weaker adhesion means infected red blood cells are less likely to clump together in people with type O, reducing the risk of the severe vascular blockage that makes malaria deadly. This selective pressure likely explains why type O blood is so common in populations with long histories of malaria exposure, particularly in parts of Sub-Saharan Africa and South America.
COVID-19 and Infection Susceptibility
During the pandemic, researchers noticed that blood type seemed to correlate with COVID-19 outcomes. Laboratory experiments found that the SARS-CoV-2 virus bound to type O red blood cells at the lowest rate of any ABO group. Cells expressing the type O antigen were less likely to be infected compared to those expressing the type A antigen. This doesn’t mean type O made anyone immune, but it appeared to provide a small, measurable reduction in susceptibility.
What Type O Means in Practice
If you have type O blood, the most practical thing to know is your transfusion profile. You can donate red blood cells to a wide range of recipients, but your options for receiving blood are limited to other type O donors. If you’re O negative, you’re in especially high demand at blood banks because of the universal compatibility of your red cells. Many blood donation services will actively recruit O negative donors because hospital demand consistently outpaces supply.
Your blood type also means you carry both anti-A and anti-B antibodies, which is why you can only receive type O red blood cells safely. On the health side, you have a modestly lower risk of heart attack, heart failure, and dangerous blood clots compared to people with other blood types, along with some degree of natural resistance to severe malaria and possibly to certain viral infections. These are population-level patterns, not guarantees, but they’re consistent findings across large studies.