Antigens and antibodies
The ABO blood group system is based on markers called antigens on the surface of red blood cells, and matching antibodies dissolved in the plasma. There are two antigens, A and B. Your antibodies are always against the antigen you do not have, so that your own blood is never attacked.
| Group | Antigen on cells | Antibody in plasma |
|---|---|---|
| A | A | anti-B |
| B | B | anti-A |
| AB | A and B | none |
| O | none | anti-A and anti-B |
Agglutination and transfusion
If donor red cells carry an antigen that meets its matching antibody in the recipient's plasma, the cells clump together — a dangerous reaction called agglutination that can block vessels. So a transfusion is safe only when the recipient's plasma has no antibody against the donor's antigens.
Example
Giving group A blood to a group B person is dangerous: the recipient's anti-A antibodies attack the incoming A antigens, causing agglutination.
Universal donor and recipient
Group O has no A or B antigens, so its red cells are not attacked by any recipient — group O is the universal donor. Group AB has no anti-A or anti-B antibodies, so it can receive cells from any group — group AB is the universal recipient. A second important system is the Rhesus (Rh) factor: people are Rh-positive if they carry the Rh antigen and Rh-negative if they do not, which matters in transfusions and in pregnancy. A person's blood group is found by mixing a drop of their blood with known anti-A and anti-B antibodies and watching which mixtures clump. Correct typing before a transfusion is life-saving, and even a universal donor's blood is cross-matched first for safety.
Remember
O gives to all (universal donor); AB takes from all (universal recipient). Match by asking: does the recipient have an antibody against the donor's antigen?