Antisera are used to determine which antigens are on the red cells. The determination of an ABO blood group is defined by demonstrating the presence or absence of antigens A and/or B on the surface of human red blood cells and by detecting the presence or absence of anti-A and/or anti-B antibodies in the plasma. It is, therefore, appropriate to identify the erythrocyte antigens using known anti-A and anti-B, then to confirm the results by verifying the presence of the corresponding antibodies in the plasma from the test blood using known red blood cells A1 and B (reverse group). Additional testing of the red blood cells with Anti-A, B reagent facilitates the recognition of certain weak subgroups and is sometimes used as further confirmation of the reactions obtained with Anti-A and Anti-B reagents.