Persons with blood group A contain within the serum of their blood an anti body which agglutinates RBC of blood group B. Persons with blood group B have an antibody against blood group A.
Individuals with blood group A can be recognized when the serum of these blood agglutinates RJBC of both blood group B and AB. Similarly individuals with blood group B can be recognized because there serum agglutinates RJBCs of blood group A and AB.