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Hello, fellow blood bankers.

I have a question about B(A) and cisAB phenotypes.

These phenotypes arise from glycosyltransferases making both A and B antigens. 

But I don't find the difference between B(A) and cisAB phenotypes.

Plus, some B(A) and cisAB alleles have the same molecular structures; for example, cisAB.05 and ABO*BA.06 have the same polymorphisms in ABO exons 6 and 7.

Could you help me understand why they are classified as different phenotypes?



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One person who I am very admired once said"There is no difference between  CisAB and A(B)"

I have the same question as Matthew.

Since they have different names and there seems no intention to change, I guess there must be something different I do not know.:)

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I agree that the difference between cisAB and B (A) is serological and divergent. They coulding be one, but they respect the names given by the authors. I do not think that the ABO * cisAB.05 and ABO * BA.06 alleles are different. It must have been an ISBT mistake!
See a summary I made in 2019.

In general, the phenotype cisAB presents normal expression of antigen A, but similarly to phenotype A2, and weak expression of antigen B. On the other hand, B(A) presents a very weak expression of antigen A, but a normal expression of B.(1)
    The rare phenotype cisAB was first described in a case of mother AB with child O.(2) Its authors suggested that this phenotype was formed by the interaction of two genes, one A2 gene and another atypical B gene, located in the same locus. However, with the molecular characterization of the cisAB-1 allele (ABO*cisAB.01), it was observed that a sequence of the ABO*A1.02 allele containing an additional mutation at position 803G>C (Gly268Ala) was capable of synthesizing a GT with mixed activity. The cisAB-1 allele is more common in Asian populations and considering the four positions that differentiate alleles A and B, it can be described as AAAB.(3) In a study of 16 Korean blood donors heterozygous for the ABO*cisAB.01 allele, it was demonstrated that both GTA and GTB have clearly decreased activity. GTA activity was 29% of GTB was 27% compared to wild GTA encoded by the A1 allele. (4) 
    Phenotype B(A) was first detected when monoclonal ABO reagents became commercially available. This phenotype exhibits normal levels of antigen B and very low levels of antigen A in tests with some anti-A monoclonal reagents. (1) The GT of this phenotype has the ability to produce normal levels of antigen B, but also use The UDP-GalNAc as substrate to produce detectable levels of antigen A.
    The B(A) alleles are variants of allele B and the first of them (ABO*BA.01) was identified by Yamamoto and collaborators. (5) This allele is commonly referred to as BABB due to the aa of position 235 being the same as consensus A1. The second allele B(A) (ABO*BA.02) has the aa sequence of allele B, being referred to as BBBB, but contains an additional mutation at position 700C>G (Pro234Ala) which is close to aa 703, one of the four that differentiate the A allele from B.(6,7) 
    By the way, a normal GTB encoded by the consensus B allele has the ability to synthesize minimal amounts of antigen A which are detectable by some anti-A reagents. As well, GTA encoded by the A consensus alleles can also synthesize minimal amounts of antigen B, which are detectable by some anti-B reagents. These reagents were considered inappropriate for the ABO phenotyping routine,(8) for example, the anti-B monoclonal antibody (BS-85), reported by Voak et al. (9)

1. Daniels G. Human blood groups: Introduction. Oxford, UK: Wiley-Blackwell2013.

2. Seyfried H, Walewska I, Werblinska B. Unusual inheritance of ABO group in a family with weak B antigens. Vox Sang. 1964;9:268-77.

3. Yamamoto F, McNeill PD, Kominato Y, Yamamoto M, Hakomori S, Ishimoto S, et al. Molecular genetic analysis of the ABO blood group system: 2. cis-AB alleles. Vox Sang. 1993;64(2):120-3.

4. Cho D, Shin MG, Yazer MH, Kee SJ, Shin JH, Suh SP, et al. The genetic and phenotypic basis of blood group A subtypes in Koreans. Transfus Med. 2005;15(4):329-34.

5. Yamamoto F, McNeill PD, Yamamoto M, Hakomori S, Harris T. Molecular genetic analysis of the ABO blood group system: 3. A(X) and B(A) alleles. Vox Sang. 1993;64(3):171-4.

6. Haslam DB, Baenziger JU. Expression cloning of Forssman glycolipid synthetase: a novel member of the histo-blood group ABO gene family. Proc Natl Acad Sci U S A. 1996;93(20):10697-702.

7.  Yu LC, Lee HL, Chan YS, Lin M. The molecular basis for the B(A) allele: an amino acid alteration in the human histoblood group B alpha-(1,3)-galactosyltransferase increases its intrinsic alpha-(1,3)-N-acetylgalactosaminyltransferase activity. Biochem Biophys Res Commun. 1999;262(2):487-93.

8. Goldstein J, Lenny L, Davies D, Voak D. Further evidence for the presence of A antigen on group B erythrocytes through the use of specific exoglycosidases. Vox Sang. 1989;57(2):142-6.

9. Voak D, Sonneborn H, Yates A. The A1 (B) phenomenon: a monoclonal anti-B (BS-85) demonstrates low levels of B determinants on A1 red cells. Transfus Med. 1992;2(2):119-27.


Edited by mpmiola
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