Concept:The blood of human beings differs in certain aspects though it appears to be similar.Various types of grouping of blood have been done.Two such groupings - the ABO and Rh are widely used all over the world. ABO grouping is based on the presence or absence of two surface antigens (chemicals that can induce an immune response) on the RBCs namely A and B.Similarly, the plasma of different individuals contains two natural antibodies (proteins produced in response to antigens).The distribution of antigens and antibodies in the four groups of blood, A, B, AB, and O. ABO blood group system: The basis of ABO grouping is two antigens:Antigen A and Antigen BThe ABO grouping system is classified into four types.It is based on the presence or absence of antigens in the red blood cells and plasma antibodies.Group A: It contains antigen A and antibody B.Group B: It contains antigen B and antibody A.Group AB: It contains both A and B antigens and no antibodies (neither A nor B).Group O: It contains neither A nor B antigens and both antibodies A and B.It is important for the blood cells to match while transfusing.Donor-recipient adaptability is needed.