The surface of the red blood cells has slots that accommodate two specific proteins – A and B. The type of proteins that fill those slots determine the blood type or the blood group of the person. A person may have only A proteins in those slots or only B proteins. Some have both A and B and some have neither. The people with neither A or B proteins on their red blood cells are said to have the blood group O.

A person with type A blood treats B proteins as intruders and harbors anti-B antibodies. Similarly, a person with type B blood has anti-A bodies. The type AB blood has antibodies for neither anti-A nor anti-B whereas type O blood has anti-A and anti-B antibodies.

The antibodies are used by the immune system to attack what it considers as intruders. Therefore, if a person with type A blood is given type B blood, the anti-B antibodies attack the B proteins and destroys those red blood cells. The red blood cells with proteins B in this case, can clump together and clog the arteries which can even prove to be fatal.

The person with both A and B proteins can receive all blood types because since his blood has both proteins, it has antibodies for neither of them. On the other hand, a person with type O blood has both anti-A and anti-B antibodies and therefore can receive only type O blood. In other words, type AB blood is universal recipient whereas type O blood is a universal donor.