Blood types explained
Two small sets of markers on your red cells, ABO and Rh, give eight blood types. Here is what each one means.
- O positive37.4% of AmericansGives to 4 · receives from 2
- O negative6.6% of AmericansGives to 8 · receives from 1
- A positive35.7% of AmericansGives to 2 · receives from 4
- A negative6.3% of AmericansGives to 4 · receives from 2
- B positive8.5% of AmericansGives to 2 · receives from 4
- B negative1.5% of AmericansGives to 4 · receives from 2
- AB positive3.4% of AmericansGives to 1 · receives from 8
- AB negative0.6% of AmericansGives to 2 · receives from 4
The ABO system
Your ABO group depends on which sugar-based antigens sit on the surface of your red cells. Type A cells carry A antigen, type B carry B, type AB carry both and type O carry neither.
Just as important is what’s in your plasma. From early childhood, you make antibodies against whichever of A and B you lack. A type O person has both anti-A and anti-B, which is why type O patients can only receive type O red cells, and why O cells, lacking both antigens, suit everyone.
The Rh system
The “positive” or “negative” is the Rh D antigen, a protein in the red cell membrane. About 85% of people have it (Rh positive). Rh-negative people don’t make anti-D automatically, only after exposure to Rh-positive blood through a transfusion or pregnancy. More on the Rh factor.
Why it matters
- Transfusion: the wrong ABO type can cause a fatal reaction within minutes. See the blood type compatibility chart.
- Pregnancy: an Rh-negative mother with an Rh-positive baby may need RhoGAM.
- Inheritance: blood types follow simple Mendelian rules. Predict a baby’s type with the blood type calculator.
Blood groups are a lovely example of a hidden system you can reason your way through: two antigens, two antibodies and one Rh switch explain every rule on this site. Play with the antigen explainer on the right, and if you like learning this way, ahaboo has narrated, interactive explainers on how other everyday things really work.
Beyond ABO and Rh
There are more than 40 recognised blood group systems (Kell, Duffy, Kidd, MNS and others). They rarely matter for a first transfusion but become important for people who receive many transfusions, such as those with sickle cell disease. Rare combinations create types like golden blood (Rh-null) and the Bombay blood type.
Frequently asked questions
How many blood types are there?
Eight in everyday use: A, B, AB and O, each either Rh positive or Rh negative. Scientifically, the International Society of Blood Transfusion recognises more than 40 blood group systems and hundreds of antigens, but ABO and Rh matter most for transfusion.
What is the most common blood type?
O positive, carried by about 37% of Americans and the most common type worldwide. A positive is a close second in the US at about 36%.
What is the rarest blood type?
Of the eight standard types, AB negative (under 1% of Americans). Outside ABO/Rh, Rh-null or “golden blood” is far rarer, with only around 50 people ever reported.
Who discovered blood types?
Karl Landsteiner described the A, B and O groups in 1901 (AB followed in 1902) and later co-discovered the Rh factor in 1940. He received the 1930 Nobel Prize in Physiology or Medicine.
Can your blood type change?
Practically, no. It is set by your genes. Rare exceptions: after a bone marrow (stem cell) transplant from a donor of another type, and temporary changes in some cancers or infections.