A1 antigen

The A1 antigen is seen in 80% of individuals who type A or AB, while the remaining 20% of patients are usually type A2, and a small minority are other weaker subgroups (<0.1%). Anti-A1 antibodies are naturally occurring IgM cold agglutinins, and are seen in 1-8% of type A2 individuals, and 22-35% of the A2B individuals. Anti-A1 antibodies typically do not cause hemolytic transfusion reactions or hemolytic disease of the fetus and newborn (HDFN). However, when the antibody is reactive at 37 degrees C, hemolysis has been reported.