The basis of ABO grouping is of two antigens- Antigen A and Antigen B. The ABO grouping system is classified into four types based on the presence or absence of antigens on the red blood cells surface and plasma antibodies.

Group A – contains antigen A and antibody B.
Group B –contains antigen B and antibody A.
Group AB –contains both A and B antigen and no antibodies (neither A nor B).
Group O – contains neither A nor B antigen and both antibodies A and B.
The ABO group system is important during blood donation or blood transfusion as mismatching of blood group can lead to clumping of red blood cells with various disorders. It is important for the blood cells to match while transfusing i.e. donor-recipient compatibility is necessary. For example, a person of blood group A can receive blood either from group A or O as there are no antibodies for A and O in blood group A.