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Questions of A More Technical Nature

The A3 Subtype


I understand there are several subtypes of A. Your last book gave some information on the A2 subtype. Can you shed some light on the others?


The A3 subgroup is much more uncommon than the A2 subgroup. The frequency ranges significantly between different ethnic groups from, 1 in 1000 of group A Danes to 1 in 150,000 French blood donors (1).

There is some genetic variability of this sub type but the most common mutation is the substition of one amino acid for another at the gene codon. (1,2,3).

It has been demonstrated that the ennzymes that produce the A3 antigen are quite variable and fit into 3 categories:

1. An enzyme with no A transferase activity.

2. An enzyme with an optimum pH of 6, which resembles the A1 producing enzyme but with only one third its activity.

3. An enzyme with an optimal pH of 7 resembling the A transferase but has very low activity (1).

The result of this is that the reactions with the anti-A typing sera are 'mixed field' (populations of agglutinated and non agglutinated cells on the same slide). The cause of this is the markedly reduced expression of A antigens on the red cells. This relates to the variation of A enzyme produced. Some A3 individuals have been observed to have only 3-4% the A antigen compared to that of A1 individuals.

Other A individuals have been demonstrated as having two distinct populations of red cells; one with A antigen and another totally devoid of the A antigen (1).

Genetically, A3s are blood group A (with regard to clotting and other disease issues). Dietwise, they are probably more similar to A non-secretors rather than secretors.

1. Daniels G Human Blood Groups. Blackwell Science Ltd 1995 Chapter 2

2. Issitt P.D. and Anstee D.J. Applied Blood Group Serology. Fourth Edition. Montgomery Scientific Publications. p218-246

3. Yamamoto F.I. Molecular Genetics of the ABO Histo-Blood group System. Vox Sang 1995; 69: 1-7.

The Ask Dr. D'Adamo internet advice column ran from 1996 to 2009, at which time Dr. D'Adamo's teaching and programming responsibilities no longer allowed him to devote time and resources to directly answering visitor questions. However we've recently reorganized this treasure-trove of material and made it again available to his readership. He occasionally posts new entries. These are marked with a NEW tag.

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