Because there are only four types of blood, it is possible to take blood from one person and donate it to another person in a process called transfusion. In order for a transfusion to work it is essential that the agglutinogens on the surface of the donor's blood cells match the agglutinogens on the surface of the recipient's blood cells. In other words, the blood type of the donor and the blood type of the person receiving the transfusion must be compatible. If the blood types don't match, special antibodies in the recipient's blood, called agglutinins, will attack the donated blood causing blood clots to form in a reaction called agglutination.

If you ever need a blood transfusion, someone will take a sample of your blood in order to determine your blood type and the genotypes of matching blood types. Someone with type A blood can receive blood from people with the AA, AO, and OO genotypes. People with type B blood can receive blood from people with the BB, BO, and OO genotypes.

There are two special genotypes when it comes to blood transfusions:
OO and AB.

The first special genotype is OO. People with O blood are said to be universal donors because they can donate blood to everybody. However, people with type O blood can only receive transfusions from other type O donors. Because O blood does not carry either the A or B agglutinogens, the immune system of a person with O blood views these agglutinogens as foreign. People with type O blood have agglutinins in their plasma that will react against the A and B agglutinogens. Therefore, type O blood will undergo agglutination if exposed to A, B, or AB blood.

The second special genotype is AB. People with AB blood are said to be universal recipients because they can receive blood from people with all four blood types. Since in AB blood both the A and B agglutinogens are presented on the surface of the red blood cells, the immune system of a person with AB blood views both of these molecules as part of itself -- not as something foreign. AB blood does not produce agglutinins against either the A or B agglutinogens, and therefore does not undergo agglutination when exposed to A, B, AB, or O blood.