The red blood cells have proteins on them that are not the same in everyone. If two people have different proteins on their red blood cells, they have different blood types.
If you have blood type A, this means you have protein A on your red blood cells. A person with blood type B has protein B on their red blood cells. If you have both protein A and B on your red blood cells, you have blood type AB. There are also people with blood type O; they have neither protein A or B on their blood cells.
Antibodies in case of wrong blood types
Before blood types were discovered, blood transfusion was often fatal. It is very important to determine a patient's blood type before a blood transfusion.
If the body makes contact with a foreign substance, it creates antibodies as a natural form of protection. Your body also creates antibodies against proteins on red blood cells that you do not have yourself, and fights off these proteins. This causes a transfusion reaction, in which the patient's body tries to break down the blood it has received. This can make the recipient seriously ill and may even be fatal.
Your body sees blood type B as foreign if you have blood type A, and vice-versa. If you have blood type AB and receive a transfusion with blood type A or B, that works fine. Your body recognizes blood type A and blood type B as the body's own. If you have blood type 0, you cannot receive blood type A, B or AB. However, you can donate blood to everyone. If you need a transfusion, your blood type is always checked first. You do not need to know what blood type you have.
Because certain blood types are more common and are thus required more often, Sanquin Blood Supply invites donors with certain blood types to donate more often than others.
In addition to the blood types A, B, AB and O, there are also other blood types that can play a role in breaking down donor blood in the recipient's body. Such as the rhesus factor. That is why a patient is tested for antibodies against these blood types before every blood transfusion.