Blood types differ based on tiny sugar molecules sitting on the surface of your red blood cells. Everyone’s red blood cells carry a base molecule called the H antigen, but what gets added on top of it determines whether you’re type A, B, AB, or O. Combined with a second factor (Rh positive or negative), these surface markers create eight common blood types that affect everything from transfusion safety to pregnancy and even disease risk.
What Makes Each Blood Type Different
The ABO system comes down to enzymes that attach specific sugars to the surface of red blood cells. If your cells add a sugar called N-acetyl-D-galactose, you have type A. If they add an extra D-galactose instead, you have type B. Type AB cells have both sugars, and type O cells have neither, leaving the base H antigen bare.
These surface sugars are called antigens, and your immune system uses them to tell the difference between “self” and “foreign.” Here’s the key: your body produces antibodies against whichever antigen your own cells lack. Type A blood carries anti-B antibodies. Type B carries anti-A. Type O carries both anti-A and anti-B. Type AB carries none. These antibodies form in the first few months of life, triggered by gut bacteria that happen to resemble blood group antigens.
This is why mismatched transfusions are dangerous. If you’re type A and receive type B blood, your anti-B antibodies attack the donated cells, triggering a potentially fatal immune reaction.
The Rh Factor: Positive vs. Negative
The second major distinction is the Rh factor, specifically the D antigen. This is a protein embedded in the red blood cell membrane, spanning it 12 times in a zigzag pattern. If your cells produce this protein, you’re Rh-positive. If they don’t, you’re Rh-negative. In most Rh-negative people of European descent, the gene that codes for this protein is simply deleted from their DNA.
Combining ABO and Rh gives you eight types: A+, A−, B+, B−, AB+, AB−, O+, and O−. Globally, O+ is the most common at roughly 40% of the population. AB− is the rarest, averaging under 0.5%. These proportions vary significantly by geography and ethnicity.
How Blood Type Is Inherited
You inherit one ABO gene from each parent. The A and B versions are co-dominant, meaning if you get one of each, both express themselves and you end up with type AB. The O version is recessive, so it only shows up when you inherit it from both parents.
This means your blood type doesn’t always reveal your exact genetic makeup. Someone with type A blood could carry either two A genes (AA) or one A and one O (AO). The same goes for type B: either BB or BO. Type AB is always one A gene and one B gene, and type O is always two O genes (oo). Two parents who are both type A, for instance, could have a child with type O if they both carry a hidden O gene.
Rh works similarly. Rh-positive is dominant, so a person can be Rh-positive while carrying one copy of the Rh-negative gene. Two Rh-positive parents can have an Rh-negative child.
Transfusion Compatibility
When it comes to receiving red blood cells, the rule is straightforward: you can’t receive blood carrying antigens your body has antibodies against. Type O− red cells carry no A, B, or Rh antigens, making O− the universal red cell donor. In emergencies where a patient’s type is unknown, O− is what gets transfused. On the other end, AB+ individuals can receive red cells from any type, making them universal recipients.
Plasma compatibility works in reverse. Since type AB plasma contains no antibodies at all, it can safely go to anyone. Type O plasma, loaded with both anti-A and anti-B antibodies, can only go to other type O recipients.
Rh Incompatibility in Pregnancy
Rh factor becomes especially important during pregnancy. If an Rh-negative mother carries an Rh-positive baby (inheriting the D antigen from the father), her immune system may recognize the baby’s blood cells as foreign and build antibodies against them. This usually isn’t a problem in a first pregnancy, but those antibodies persist. In a second Rh-positive pregnancy, they can cross the placenta and attack the baby’s red blood cells.
The standard prevention is an injection of Rh immune globulin, typically given around 28 weeks of pregnancy and again within 72 hours of delivery. This treatment works by clearing any fetal red blood cells from the mother’s system before her immune system has a chance to react. If the injection is missed in that window, guidelines recommend giving it as soon as possible, up to 28 days after delivery.
Blood Type and Disease Risk
Your blood type appears to influence susceptibility to a range of conditions, likely through its effects on clotting and inflammation.
- Heart disease and clotting: Types A, B, and AB are associated with a greater risk of heart attack from coronary artery disease compared to type O, with AB carrying the highest risk. These same non-O types are linked to higher rates of clotting disorders, which likely explains the connection.
- Stroke: People with type A blood are slightly more likely to have a stroke before age 60 than those with type O, possibly because of differences in clotting factors.
- Stomach cancer: Type A is associated with higher rates of H. pylori infection, which can cause ulcers and inflammation that sometimes progress to stomach cancer.
- Other cancers: Types A, B, and AB show a modestly elevated risk for lung, breast, colorectal, cervical, and pancreatic cancers compared to type O.
- Malaria: Mosquitoes appear to prefer type O blood in lab experiments, but type O actually offers some protection against the most severe effects of malaria. This trade-off is one reason type O is so common in regions where malaria is prevalent.
- COVID-19: A large study of European patients found that type O was associated with a slightly lower risk of dying from COVID-19.
These are population-level patterns, not destiny. The increased risks are generally modest, and lifestyle factors like diet, exercise, and smoking have a far larger influence on your health than your blood type does.
Why Blood Type Diversity Exists
The persistence of multiple blood types across human populations is a textbook case of evolutionary balancing. Different blood types offer advantages against different infectious diseases. The geographic distribution of ABO types tracks closely with the historical prevalence of specific pathogens, particularly the malaria parasite and cholera. Norovirus and rotavirus also interact differently with blood group antigens, meaning no single type is universally “best.” This pressure from multiple pathogens in different regions has kept all the major blood types circulating in the human gene pool.
Beyond ABO and Rh
ABO and Rh are the two systems that matter most in everyday medicine, but they’re far from the whole picture. The International Society of Blood Transfusion currently recognizes 48 distinct blood group systems, each defined by different proteins or sugars on red blood cells. These include systems like Kell, Duffy, and Kidd, which can become relevant for people who receive frequent transfusions (such as those with sickle cell disease) and gradually develop antibodies against these rarer antigens. For most people, though, knowing your ABO type and Rh status covers what you need for transfusions, pregnancy planning, and understanding your baseline health profile.