The human blood is classified under different groups with different types of classification. Some of these classifications are based on protein substances found in the blood.
There are 35 known human blood group system but the two commonly used type are the ABO blood grouping system and the Rhesus factor grouping system.
The ABO Blood Group
The ABO blood grouping system was first identified by Karl Landsteiner, an Austrian immunologist, in 1901. The ABO blood group is classified by the presence of specific antigens on the surface of the red blood cell. The presence of these antigens is an inherited character of the red blood cell.
There are two types of antigen that could be present on the surface of the red blood cell and it is the A and B antigen. If the surface of the RBC has antigen A, it is said to belong to blood group A.
The presence of antigen B means the blood falls under blood group B. the presence of both antigen means the blood falls under the blood group AB and the absence of both antigen means the blood falls under blood group O.
When the RBC has any of the antigen on the blood surface, the antibody of the opposite antigen is found in the serum and this is the basis for blood compatibility in blood transfusion.
The presence of an antibody in the serum means that if the corresponding antigen is found in the body, there would be an agglutination reaction because the antibody is against it and this may lead to serious dangerous conditions.
Example, A group blood has anti-B antibodies in the serum, because of this an individual of blood group A cannot receive blood from a B or AB group because of the presence of antigen B on their blood which will be attacked by the anti-B antibody of the person of blood group A. likewise, B blood has anti-A antibody in the serum and cannot receive blood from either blood group A or AB.
AB has no antibody in the serum and can receive blood from all the blood group types because the antigen wouldn’t be attacked while blood group O, which has neither antigen, will have both anti-A and anti-B antibody in its serum, and this makes them unable to receive blood from any other group besides blood group O.
Because of the above explanation, AB are referred to as universal recipients, because they can receive blood from all other blood groups, while blood group O has been termed the universal donor because they can donate to all blood groups. Blood group O is the most common blood type in the world.
The Rhesus Blood Group System
The Rhesus blood group system is the second most important blood group system among the 35 known blood group system, after the ABO grouping system. This system is also defined by the presence of antigens on the surface of RBC.
The Rh system consists of 50 defined blood group antigens among which 5 are the most important. These five are the antigens D, C, c, E, and e. however, the common classification of Rh blood group is based on the D antigen. The terms Rh positive and Rh negative refer to whether the Rh Antigen D is present on the surface of the blood or not.
However, other Rh antigens on the blood are clinically relevant. Apart from being important in compatibility for blood transfusion, the D antigen also determines the risk of erythroblastosis fetalis (hemolytic disease of the new born) in pregnant women and their babies.
The Rh factor is usually associated with the ABO blood group system when one is determining their blood group. It is usually indicated by the Rh positive or Rh negative suffix to the ABO blood type. Rh+ does have the antigen present whereas Rh- does not have the antigen present.
Unlike the ABO grouping system, immunization against the Rh factor in Rh- individuals can generally only occur through blood transfusion or placental exposure in pregnant women. Therefore, a person that is said to be A+ has both the antigen A, based on the ABO system, and the Rhesus Antigen D based on the Rhesus system, present on the surface of their red blood cells.
Likewise, a person said to be A- has only the antigen A and not the Rhesus Antigen D on the surface of their red blood cells.
If Rh+ blood is transfused to someone that is Rh-, the body of the Rh- person will develop antibodies against the Rh factor after the first transfusion and most times there is no reaction. But the subsequent transfusions will be dangerous because antibodies have already been developed against the Rh factor.
Because of this, people with blood group O- are the real universal donors while people of AB+ blood group are the actual universal recipients.