The classification of blood types is based on the different types of antigens present on the surface of the red blood cells (RBC).
The human blood group is broadly classified into four groups called A, B, AB and O. The letters stand for the type of antigen present on the red blood cells.
The corresponding antibodies are carried in the plasma and if the person has a particular antigen in his red cells, he cannot have the corresponding antibody, since agglutination would occur. Thus group A contains antigen A and antibody anti-B.
Similarly group B contains antigen B and antibody anti-A. Group AB contains antigens A and B and no antibodies of either type. Group O has no antigens and antibodies anti-A and anti-B.
There is another factor behind blood grouping. It is a type of blood protein, first discovered in the blood of the Rhesus monkey.
Later it was discovered that some human beings also posses this factor in their blood.
If this factor is present in the blood cells, then the blood cells are called Rh positive and if they are absent it is called Rh negative.
