Rare subgroups of A also exist. These comprise less than 1% of the total pool of A genes. Of these rare types, A3 is the most common, but Ax, Aend, and Ael have also been identified.
In subgroup A3, the red blood cells characteristically give a “mixed field” agglutination pattern when tested with anti-A and anti-A,B. Small clumps of agglutinated cells are present among large numbers of cells, which absorb the antibody to their surface but are not agglutinated by it. Mixed field reactions or weak reactions should be further investigated before the ABO type is interpreted.
