Every person belongs to one of eight blood groups. These blood groups, or blood types, are sub-types of the four larger ABO blood groups – A, B, AB or O. The Rhesus (Rh) system further divides these four groups into either Rh+ or Rh-, leaving eight blood groups: O-, O+, B-, B+, A-, A+, AB- or AB+.
ABO blood grouping is determined by the correlation of visible clumping in mixtures of plasma, the liquid component of blood, and the red cells, which carry oxygen.
The Rhesus system, first discovered in 1939, relies on the presence of antigens – the D antigen in particular. Antigens are molecules that bind to an antibody. A person either has or hasn’t got the Rh factor (D antigen) on the surface of their red cells, indicating a positive (Rh+) or negative (Rh-).
Not all blood types are compatible. This is due to the differences in antibodies that occur in different blood types. While blood group A individuals have naturally occurring anti-B blood group antibodies in their plasma, blood group B has anti-A blood group antibodies and blood group O has anti-A and -B blood group antibodies.
So, for example, if a blood group B patient is given blood group A, the anti-A antibodies in their plasma will destroy the red cells in the transfused unit and lead to severe complications or even death.
Blood group O individuals are known as “universal donors”. They lack A and B blood group antigens which makes it possible for their blood to be given to all ABO types.
AB+ blood group individuals on the other hand are known as “universal recipients”. They lack naturally occurring anti-A and -B and can receive all ABO groups.
Occasionally an individual is born with an unusual, specific red-cell antigen, or without an antigen that is common to most people. These differences are recognised as rare blood types and are difficult to match with regular blood groups due to antibody presence.
That is why compatibility tests on the blood of the patient and the donor are essential before every blood transfusion. It ensures that the recipient will not experience an adverse reaction.