Blood Types Explained
Human blood is sorted into types based on antigens on red blood cells and antibodies in plasma. Two systems, ABO and Rh, combine to give the eight common blood types.
The ABO Blood Group System
The ABO system sorts blood into four groups by which antigens sit on the red blood cell surface:
| Group | Antigens on red cells | Antibodies in plasma |
|---|---|---|
| A | A antigen | Anti-B |
| B | B antigen | Anti-A |
| AB | A and B antigens | None |
| O | None | Anti-A and Anti-B |
The Rh Factor
The Rh system adds one more split, based on the Rh D antigen:
- Rh positive (+): the Rh D antigen is present on red cells.
- Rh negative (−): the Rh D antigen is absent.
Combining ABO (4 groups) with Rh (2 states) produces the 8 common blood types.
The 8 Blood Types
Universal Donor and Universal Recipient
O− carries no A, B, or Rh antigens, so its red cells can be given to anyone. It is the universal red cell donor. AB+ carries no anti-A, anti-B, or anti-Rh antibodies, so it can receive red cells from anyone. It is the universal recipient. See the full blood type compatibility chart for every pairing.
How Blood Typing Works
To find a blood type, a sample is mixed with anti-A, anti-B, and anti-Rh (anti-D) reagents. Wherever the blood clumps (agglutinates), the matching antigen is present. The pattern reveals the ABO group and Rh status. You can try this yourself in the Blood Typing Game.
The Nobel Prize Connection
In 1930, Karl Landsteiner won the Nobel Prize in Physiology or Medicine for discovering human blood groups in 1901. In 1940 he helped identify the Rh system. Together these discoveries made blood transfusions safe and have saved countless lives.