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A2 Subgroup and Anti-A1 Antibody

The A blood group can be subdivided into A1 and A2. A1 phenotype is more common in all populations. Approximately 22% of Group A individuals have the A2 subgroup. The A1 and A2 genes code for different A transferases. The A2 transferase is less efficient in converting H to A substance, resulting in red cells that have approximately 20-25% less A antigen than A1 cells. In addition to this quantitative difference, A1 and A2 antigens have a different carbohydrate composition. This biochemical difference may explain why 1-8% of A2 individuals and 22-35% of A2B individuals produce anti-A1 antibody. Dolichos biflorus lectin will agglutinate A1 and A1B but not A2 or A2B red cells.

The presence of anti-A1 may cause discrepancies in forward and reverse grouping. A patient will forward type as group A and reverse type as a group O.

Forward Type

Reverse Type



A cells

B cells






Anti-A1 is considered clinically significant when it reacts at 37 C. In these cases, patients should be transfused with red blood cells that are compatible after a full crossmatch (group O or group A sub).

Svensson L etal. Blood group A1 and A2 revisited: an immunochemical analysis. Vox Sang 2009;96:56-61.


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