Subgroups of ABO System and Bombay Phenotype

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For practical purposes, the ABO blood group is divided into
A, B, AB and O blood groups. Only when problems arise
subgroups are looked into and distinction of A and B subtypes
is made.
SUBGROUPS OF ABO SYSTEM
There are many subgroups of ABO system. They are mentioned
below:
Group A: The important subgroups of A are A1 and A2.
The other subgroups are: A3, Ax, Am, Ay Ael, Aend, Abantu,
Afinn, and Aint.
The weaker variants are: A2, A3, Ax, Am, AyAel, Aend, Abantu,
Afinn, and Aint.
Group B: The subgroups are: B, B3, Bx, Bm, Bel, the weaker
variants are: B3, Bx, Bm, Bel.
Clinically Important Subgroups
The subgroups of blood group A having clinical importance
are A1 and A2.
Serological differences between A1 and A2 are based on
reactivity with human anti-A1 or anti-A1 lectin from Dolichos
biflorus.
Amongst A blood group, 80% of population belongs to A1
blood group and about 20% belongs to A2 blood group.
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Subgroups of ABO System
and Bombay Phenotype
55

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56
Essentials of Blood Banking and Transfusion Medicine
About 1–8% of the A2 blood group individuals have anti-
A1 antibodies.
Also about 22–35% of the A2B blood group individuals have
anti-A1 antibodies.
A2 blood group has diminished enzyme activity and weak
expression.
A2 if misdiagnosed as O blood group and when given to
O blood group patients, the anti-A and anti-B antibodies
present in the patient’s serum will destroy the transfused
blood.
BOMBAY PHENOTYPE (BOMBAY BLOOD GROUP)
Individuals with the rare Bombay phenotype (hh) do not
express antigen H on their red blood cells. As H antigen serves
as precursor for producing A and B antigens, the absence of
H antigen means the individuals do not have A or B antigens
as well (similar to O blood group). However, unlike O group,
the H antigen is absent, hence the individuals produce
isoantibodies to antigen H as well as to both A and B antigens.
In case they receive blood from O blood group, the anti-H
antibodies will bind to H antigen on RBC of donor blood and
destroy the RBCs by complement-mediated lysis. Therefore,
Bombay phenotype can receive blood only from hh donors
(Bombay blood group).
Thus, in this blood group:
• The red cells lack H, A and B antigen.
• It was first discovered in Bombay in 1952 by Dr YM Bhende
and colleagues.
• Symbol Oh denotes the phenotype, on routine ABO typing
tests Bombay blood group mimic those of group O persons.
• Oh serum contains strong anti-H, anti-A and anti-B. When
serum from the Oh is tested against group O red cells, after
immediate-spin strong agglutination and/or haemolysis
occurs.
• This blood group occurs 1 in 13000 population all over the
world.

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Subgroups of ABO System and Bombay Phenotype
57
Para-Bombay Phenotypes
• Ah, Bh and ABh red cells lack serologically detectable
H antigen but carry small amounts of A and/or B antigen,
depending on the individual’s genes at the ABO locus.
• Tests with anti-A or anti-B reagents may or may not give
weak reactions but the cells are non-reactive with anti-H
lectin or with anti-H serum from Oh persons.
• Para-Bombay phenotypes have a non-functional H gene but
a normal Se gene and thus will express A, B and H antigens
in their plasma and secretions.
• The sera of Ah and Bh people contain anti-H in addition to
the expected anti-A or anti-B antibodies.