Two subgroups of A1 and A2 were initially discriminated by von Dungern in 1911. Dolichos biflorus plant lectin or the anti-A1 antibody that was prepared by pre-treating serum from blood group B individuals with A2 RBCs agglutinates A1 RBCs, whereas no agglutination was observed with A2 RBCs. Later studies have found that the number of A antigens is much higher with A1 RBCs than A2 RBCs. It was also shown that A1 RBCs possess A antigens on the branched and unbranched core structures, whereas A2 RBCs have antigens predominantly on unbranched structures. Additionally, more recent studies have shown the presence of A antigens on the type 4 core structure only on A1 RBCs.
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