DIFFERENTIATION OF THE A1 AND A2 SUBGROUPS OF THE AB0 SYSTEM: BIOLOGICAL BACKGROUND AND SEROLOGICAL STRATEGY

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Introduction. The identification of weak variants of the A antigen, as well as their differentiation, is necessary for the proper selection of erythrocyte-containing media for blood transfusions. To this end, selective anti-A1 reagents that react only with the A1 antigen are used in combination with anti-A reagents reacting equally with the A1 and A2 antigens. Given that the expression of the A antigen varies within the subgroups and there is no established standard for reagents and procedures, the interpretation of the obtained results presents difficulties.Aim. To develop a strategy for identifying the variants of the A antigen using available reagents in an agglutination reaction.Methods. We compared the effectiveness of four anti-A1 and two anti-H reagents using 23 blood samples (groups A2 and A2B) and control samples (groups A1 and A1 B). Two types of anti-A1 reagents were employed: Dolychos biflorus lectin and monoclonal antibodies. All of the reagents were designed for direct agglutination reactions. Belonging of the erythrocytes to the A2 subgroup was confirmed using genetic analysis.Results. It is shown that anti-A1 reagents did not interact with A2B red blood cells and often reacted with A2 red blood cells. The strength of the reaction with A2 red blood cells varied greatly and was weaker than with A1 red blood cells; however, it hindered the subgroup identification. Simultaneous tests conducted using an anti-H reagent allowed the authors to draw an unambiguous conclusion about blood belonging to a subgroup: a strong reaction indicated the A2 subgroup, whereas a negative or weak reaction indicated the A1 subgroup. A discrepancy was noted between the results obtained for two donors using serological and molecular methods: the A3 subgroup was identified serologically, whereas genotyping revealed the AB0*A1 allele. In both cases, direct sequencing showed a combination of mutant alleles giving the A3 phenotype. When using commercial kits to perform genotyping analysis through a polymerase chain reaction, it should be taken into consideration that primers are matched to the most common variants and cannot detect all mutations of the AB0 gene.Conclusion. Reliable identification of the A2 subgroup through serological methods is possible when using lectin or monoclonal anti-A1 antibodies in combination with a monoclonal anti-H reagent.Conflict of interest: the authors declare no conflict of interest.Financial disclosure: the study had no sponsorship.