The ABO system classifies blood into four major blood groups. These blood groups are determined by the presence or absence of two antigens – A and B – on the surface of red blood cells:
*.Group A – only the A antigen is present on red cells (and B antibody in the plasma)
*.Group B – only the B antigen is present on red cells (and A antibody in the plasma)
*.Group AB – has both A and B antigens on red cells (but neither A nor B antibody in the plasma)
*.Group O – has neither A nor B antigens on red cells (but both A and B antibody are in the plasma.
Rhesus blood groups (or the +/- one often notices next to the ABO blood group) are more complicated than the ABO system, but an important part is the D antigen.
*.Rhesus Positive-has the D antigen on the surface of red blood cells
*.Rhesus Negative-does not have D antigen on the surface of red blood cells (has D antibody in the plasma)
The antibodies will agglutinate with the corresponding antigens if they are present within the blood. For group O there shouldn’t be any agglutination within the A or B circles because the red blood cell does not have A or B antigens.