Consequently, successful blood transfusions require the "matching" of donor and recipient blood type. These are proteins that will attack the antigens on the surface of red blood cells of a different blood type. Paired with the red blood cell antigens, your plasma also contains a specific set of antibodies. They can also be determined by antibodies in the blood plasma. The blood groups A, B, AB or O can be detected by the presence or absence of antigens on the surface of the red blood cells. A blood type is considered rare when more than 200 donors have to be screened to find one compatible donor.
A small percentage of people have rare blood types, which appear as an unusual and sometimes extensive series of letters in addition to their ABO type. This varies among different ethnic populations. If the red blood cell has neither antigen, the blood is called "type O". If the red blood cell has both A and B antigens, the blood is called "type AB". If the red blood cell has B antigen only, that blood is called "type B". In the ABO system, one antigen is labeled "A" and the other "B." If the red blood cell has only A antigen on it, that blood is called "type A". Each person's blood contains a specific and inherited set of these.
There are two distinct antigens (a type of protein) present on the surface of some people's red blood cells. This is because if you lack some of these molecules on your red blood cells, you make antibodies against that molecule. When scientists mixed different people’s red cells, they noted that the plasma from certain people made some red cells agglutinate or cluster together. In blood transfusion the two most important group systems are the ABO-system and the Rhesus system. There are more than 45 known blood groups on red blood cells – not all of them are clinically significant. These molecules were studied and divided into different group systems according to their structure. There are millions of different molecules on our red cells, each with its own function.