Blood Type Compatibility Chart
Select a blood type to see who can donate to you and who you can donate to, based on ABO and Rh factor. Includes a full 8×8 compatibility matrix and population frequency data. No signup, runs entirely in your browser.
A+
Second most common blood type — about 30% of the population.
O+
Most common blood type — about 38% of the population.
Full Compatibility Matrix
| Donor ↓ / Recipient → | A+ | A- | B+ | B- | AB+ | AB- | O+ | O- |
|---|---|---|---|---|---|---|---|---|
| A+ | ✓ | ✗ | ✗ | ✗ | ✓ | ✗ | ✗ | ✗ |
| A- | ✓ | ✓ | ✗ | ✗ | ✓ | ✓ | ✗ | ✗ |
| B+ | ✗ | ✗ | ✓ | ✗ | ✓ | ✗ | ✗ | ✗ |
| B- | ✗ | ✗ | ✓ | ✓ | ✓ | ✓ | ✗ | ✗ |
| AB+ | ✗ | ✗ | ✗ | ✗ | ✓ | ✗ | ✗ | ✗ |
| AB- | ✗ | ✗ | ✗ | ✗ | ✓ | ✓ | ✗ | ✗ |
| O+ | ✓ | ✗ | ✓ | ✗ | ✓ | ✗ | ✓ | ✗ |
| O- | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ |
Blood Type Biology
ABO antigens
Blood type is determined by antigens on the surface of red blood cells. Type A has A antigens, type B has B antigens, AB has both, and O has neither. Antibodies in plasma attack any foreign antigen present.
Rh factor
The Rh (Rhesus) factor is a separate antigen. Rh+ individuals have it; Rh− individuals do not. Rh incompatibility is critical in pregnancy — if an Rh− mother carries an Rh+ foetus, she can develop antibodies that attack future Rh+ pregnancies.
Inheritance
Blood type follows codominant inheritance. If one parent is type A (IA IO) and the other is type B (IB IO), children can be A, B, AB, or O. The I gene has three alleles: IA, IB (codominant), and IO (recessive).
Why O is most common
Blood type O is most common worldwide (about 44% of people are O+). It evolved before A and B mutations. Type A and B antigens are thought to have co-evolved with pathogens — different allele frequencies persist in different populations.
Frequently Asked Questions
What are the 8 blood types?
The ABO system has four groups (A, B, AB, O) and the Rh factor adds positive (+) or negative (−), giving 8 types: A+, A−, B+, B−, AB+, AB−, O+, O−.
Who is the universal donor?
O− (O negative) is the universal red blood cell donor because it lacks A, B, and Rh antigens, so any recipient can receive it without an immune reaction. It is used in emergency transfusions before blood typing.
Who is the universal recipient?
AB+ (AB positive) is the universal recipient for red blood cells — their immune system does not produce antibodies against A, B, or Rh antigens, so they can receive any blood type.
Why does blood type compatibility matter?
If incompatible blood is transfused, the recipient's antibodies attack the donor's red blood cells (agglutination and haemolysis). This can be life-threatening — causing acute haemolytic reactions, kidney failure, and death.
Can AB− donate plasma to anyone?
For plasma, compatibility is reversed: AB plasma is the universal donor because AB individuals have no anti-A or anti-B antibodies. For red cells the rules described above apply.
Is this tool for medical use?
No. This tool is for educational purposes only. Actual blood transfusion decisions are made by medical professionals following rigorous testing including crossmatching, which checks individual compatibility beyond ABO and Rh.