The ABO Grouping is defined both by the presence of 'A' and/or B' antigens on the surface of the red blood cells and by the simultaneous presence of anti-A and/or anti-B antibodies in the serum. An individual has in his serum the antibodies corresponding to the antigens which are not on his red blood cells. It is thus necessary to identify the erythrocyte antigens by the known ANTI-A, ANTI-B and ANTI-AB monoclonal antibodies(forward typing) and then confirm the results by verifying the presence of corresponding antibodies in the blood serum to be tested with control cells:’A1,‘A2, ‘B’ and ‘O’ (Reverse typing). Human red blood cells possessing 'A' and /or 'B' antigen will agglutinate in the presence of antibody directed towards the antigen. For example agglutination of RBCs with Anti–A as well as with Anti-B monoclonal antibodies indicates the presence of both ‘A’ & ‘B’ antigen on the red blood cells.
