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

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.

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