Blood types are a classification system based on specific molecules sitting on the surface of your red blood cells. There are eight common blood types created by two overlapping systems: ABO (A, B, AB, or O) and Rh (positive or negative). Your blood type is determined at conception, inherited from your parents, and never changes throughout your life.
How the ABO System Works
Every red blood cell is coated with sugar molecules called antigens. The ABO system comes down to which sugars your cells carry. Type A blood has A antigens, type B has B antigens, type AB has both, and type O has neither. The “O” essentially means your red blood cells display a bare precursor molecule with no A or B sugar added to it.
Your immune system treats any ABO antigen it doesn’t recognize as a threat. This means your body automatically produces antibodies against whichever antigens your own cells lack. If you’re type A, your blood contains anti-B antibodies. If you’re type B, you carry anti-A antibodies. Type O individuals have both anti-A and anti-B antibodies circulating in their blood. Type AB individuals, carrying both antigens, produce neither antibody.
This antibody system is what makes blood type matter in transfusions. If type A blood enters a type B person’s body, the recipient’s anti-A antibodies attack the donated red blood cells, triggering a potentially fatal reaction.
What Rh Positive and Negative Mean
The second part of your blood type, the plus or minus sign, refers to a protein called the D antigen on the surface of your red blood cells. If it’s present, you’re Rh-positive. If it’s absent, you’re Rh-negative. Combining ABO with Rh gives you the eight familiar types: A+, A−, B+, B−, AB+, AB−, O+, and O−.
How common Rh-negative blood is varies dramatically by ethnicity. Among white non-Hispanic Americans, about 17% are Rh-negative. In Asian populations, that figure drops to around 1.7%. In India, roughly 6% of the population is Rh-negative, while in Tanzania, only about 2% are.
How You Inherit Your Blood Type
You receive one ABO gene from each parent, giving you two copies. The A and B versions of the gene are codominant, meaning if you inherit one of each, both are active and you end up with type AB. The O version is recessive, producing a nonfunctional enzyme that doesn’t add any sugar to red blood cells. So a person with type A blood might carry two A genes, or one A and one O. Two parents who are both type A can have a type O child if they each pass along their hidden O gene.
Rh works similarly. You can be Rh-positive while carrying one copy of the gene that codes for no D antigen, which means two Rh-positive parents can have an Rh-negative child.
Who Can Donate to Whom
For red blood cell transfusions, compatibility depends on avoiding antigens that the recipient’s immune system will attack. The rules break down like this:
- Type O red blood cells can go to anyone (O, A, B, or AB recipients), making O the universal red cell donor.
- Type A can receive red cells from types A and O.
- Type B can receive from types B and O.
- Type AB can receive red cells from all four types, making AB the universal red cell recipient.
Plasma transfusions flip this logic. Plasma contains antibodies rather than antigens, so the danger comes from donor antibodies attacking the recipient’s cells. Type AB plasma contains no anti-A or anti-B antibodies, making it the universal plasma donor. Type O plasma, loaded with both antibodies, can only go to other type O recipients.
Blood Type Distribution Around the World
Type O is the most common blood type globally, though its prevalence varies by region. In the United States, it ranges from about 40% among Asian Americans to over 56% among Hispanic Americans. In China, O accounts for roughly 36% of the population, with A close behind at 32%. India shows an unusual pattern where types O and B are nearly tied at about 35% and 34%, respectively. In parts of sub-Saharan Africa, O can exceed 50%.
Type AB is consistently the rarest of the four ABO groups everywhere in the world, typically making up between 3% and 8% of any given population. Turkey is an outlier in a different way: type A is the most common group there at nearly 44%, one of the highest rates recorded.
Rh Incompatibility in Pregnancy
Rh factor takes on special importance during pregnancy. If an Rh-negative mother carries an Rh-positive baby, small amounts of the baby’s blood can cross the placenta and enter her bloodstream. Her immune system may then produce anti-D antibodies that treat the baby’s red blood cells as foreign invaders. In a first pregnancy this rarely causes harm, but those antibodies persist. In a subsequent pregnancy with another Rh-positive baby, the antibodies can cross the placenta and destroy the baby’s red blood cells, causing a condition called hemolytic disease of the newborn.
To prevent this, Rh-negative mothers receive an injection of anti-D immune globulin around weeks 26 to 28 of pregnancy and again within 72 hours after delivering an Rh-positive baby. This treatment works by suppressing the mother’s immune response before she forms her own permanent antibodies. If the injection is missed at delivery, partial protection is still possible if given within 13 days, and it may offer some benefit up to 28 days after birth.
Blood Type and Disease Risk
Your blood type isn’t just relevant during transfusions and pregnancy. A growing body of evidence links ABO type to varying risks for several diseases, with type O generally coming out ahead.
People with non-O blood types have roughly 25% higher concentrations of certain clotting factors in their plasma. This translates to a greater risk of blood clots, ischemic heart disease, and stroke. The flip side is that type O individuals are more prone to excessive bleeding, since their clotting factor levels run about 25% lower.
For cancer, the associations are modest but consistent across large studies. Type A individuals show elevated rates of stomach cancer (about 11% higher odds than type O), and the connection strengthens in people infected with certain strains of the bacterium that causes stomach ulcers. Pancreatic cancer risk runs highest in type B (72% increased odds compared to type O), followed by AB (51% higher) and A (32% higher).
Type AB carries the highest observed risk for type 2 diabetes, with one analysis showing nearly double the odds compared to type O after adjusting for other metabolic factors. A large prospective study also found that AB individuals had an 82% increased risk of cognitive impairment, independent of age, race, or geographic region.
These are statistical associations across populations, not certainties for any individual. Many other factors, from diet and exercise to genetics unrelated to blood type, play a far larger role in whether any of these conditions develop.
Rare and Unusual Blood Types
Beyond the eight common types, there are dozens of additional blood group systems and some extraordinarily rare variants. The most famous is the Bombay phenotype, found primarily in people of South Asian descent. Individuals with this phenotype lack the foundational molecule (called the H antigen) that A and B sugars are built on. Their red blood cells carry no A, B, or H antigens at all. Standard blood typing may misidentify them as type O, but they are critically different: they produce antibodies against H, A, and B, meaning a transfusion of even type O blood could trigger a severe reaction. They can only safely receive blood from other people with the Bombay phenotype.
Even rarer is Rh-null blood, sometimes called “golden blood,” which lacks all Rh antigens on the red cell surface. Fewer than 1 in 6 million people have it. Rh-null red blood cells can theoretically be transfused to anyone within the Rh system, making them extraordinarily valuable, but the tiny number of known donors worldwide makes this more of a scientific curiosity than a practical resource. People with Rh-null blood also experience a trade-off: their red blood cells are structurally fragile, leading to chronic mild or moderate anemia as the cells break down faster than normal.