An individual's ABO group is determined by testing the red blood cells with reagent anti-A and anti-B sera.
Agglutination of the individual's red cells by the appropriate antisera signifies the presence of the antigen on
the red cell while no agglutination with the antisera signifies its absence.
Additional testing of the red cells with Anti-A,B sera may be used for the detection of weak subgroups of A
or B. Because weak A or B subgroups have fewer antigens present on the red blood cells, very weak or
negative reactions may occur with anti-A or anti-B serum., but will give stronger or positive reactions with
anti-A,B serum. The use of potent moncolonal antibodies has dramatically decreased the use of anti-A,B.
In the past Anti-A sera was obtained from group B individuals, anti-B serum was obtained from group A
individuals, and anti-A,B serum was obtained from group O individuals. Most facilities currently use
monoclonal source antibodies for blood grouping.