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ABO blood group System
By : Mr. Waggas Ela’as, M.Sc; MLT
ABO BLOOD GROUP SYSTEM
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Discovered by Karl Landsteiner; locus on chromosome 9
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Single most important blood group for the selection and
transfusion of blood
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Widely expressed  tissues & body fluids including red
cells, platelets & endothelial cells
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Three antigens: A, B, H
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Two major antibodies: anti-A and anti-B
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Four phenotypes: A, B, AB, O  A & B Ag’s autosomal codominant (expressed on group A, B and AB red cells; O
phenotype autosomal recessive (most frequent)
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People inherit H gene which will convert a “precursor
Substance” into an H Substance. If the person –then – inherit
A gene ; it will convert the H substance into A antigen. If he
inherit B gene ; it will convert the H substance into B antigen. If
he inherit both A & B genes ; they will convert the H
substance into AB antigens. If he inherit NO A nor B genes;
the H substance will remain unconverted. This is named “group
O”.
If the person didn’t inherit the H gene; he will not have an H
substance. This is named “Bombay group”.
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The A and B genes control the synthesis of specific enzymes
(glycosyl-transferase) responsible for the addition of single
carbohydrate residues (N-acetyl galactosamine for group A
and D-galactose for group B) to a basic antigenic glycoprotein
or glycolipid with a terminal sugar L-fucose on the red cell,
known as the H substance. The O gene is an amorph and
does not transform the H substance.
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The H gene is converted to H substance. Subsequently the H
substance is acted upon by specific transferases which is
converted to either A or B antigen.
Some H substance remains unconverted and is expressed
as H antigen. Since there is no conversion of H substance
in O blood group, the amount of H antigen is maximum on
red cells (The O gene is a silent allele).
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ABO BLOOD GROUP SYSTEM
ABO Antigens
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Present on the surface of red cells as well as tissue and
endothelial cells in the body (found on all the cells of the body
tissues, hence the ABO compatibility is a prerequisite in cases of organ
transplants).
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Found in soluble form in plasma & other body secretions
in people known as secretors.
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Inherited in simple
individual’s parents.
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3 possible genes that can be inherited: A, B, O
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A and B genes produce a detectable product
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O gene does not produce a detectable product
Mendelian
fashion
from
an
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ABO BLOOD GROUP SYSTEM
Phenotype
Antigen
Natural
antibody
Genotype
A
A only
Anti-B
AA or AO
B
B only
Anti-A
BB or BO
AB
A and B
None
AB
O
None
Anti-A,
Anti-B
OO
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ABO BLOOD GROUP SYSTEM
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ABO BLOOD GROUP SYSTEM
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ABO BLOOD GROUP SYSTEM
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ABO BLOOD GROUP SYSTEM
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H Antigen
• required to produce either A or B antigens
• possible genetic combinations: HH, Hh, or hh
• HH or Hh (+)  produce H Ag  99.99% of
Caucasians
• hh  does not produce H Ag  Bombay phenotype
(Oh)
• anti-H antibodies rare – found only in individuals
with Bombay phenotype
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Example of determining offspring blood types from known
or suspected genotypes:
Genotype parent #1 (AO)
Genotype parent
#2 (AB)
A
O
A
AA
AO
B
AB
BO
Phenotypes of possible offsprings: A, AB, B
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Frequencies of ABO Blood Groups:
Blood Group
Frequency
O
45%
A
41%
B
10%
AB
4%
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ABO Subtypes:
[Subgroups of A are of minor clinical significance].
1. A variants (A1, A2)
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A1 most common (80%) & most antigenic
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A1 and A2 differentiated using antisera specific
for A1 Ag (anti-A1 lectin) prepared from seed
known as Dolichos biflorus  (+) reaction with
A1 but not A2
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Anti-A  reacts with both A1 & A2 but more
strongly with A2
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A2 red cells have fewer A antigen sites than A1 cells. In
addition, the number of A antigens on A1 RBCs is
approximately five times more than on A2 RBCs.
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The H antigen content of red cells depends on the ABO
group and when assessed by agglutination reactions with
anti-H, The strength of reaction tends to be graded O > A2 >
A2B > B > A1 > A1B.
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Other subgroups of A are occasionally found (e.g., A3, Ax)
that result from mutant forms.
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ABO Subtypes:
2. Weak A and weak B phenotypes
3. Null phenotypes:
(a) Bombay (Oh)
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No A, B or H Ag on red cells & secretions
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With anti-A, anti-B & anti-H in their sera
(b) para-Bombay
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Absent or only trace A,B & H Ag’s detected on
RBCs with normal expression in secretions &
body fluids
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ABO Antibodies
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Natural antibodies  antigenic stimulus
environmental  exposure occurs from birth
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Newborns  without ABO antibodies of their own;
begin to produce Abs with detectable titer at 6
months of age
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Other characteristics of ABO antibodies:
is
1. IgM :(complete Abs).
2. Reacts at room temp. after an immediate spin
(cold Abs : react best at 4oC, but can react also
at 37oC).
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Summary
Precursor substance
No H gene
Bombay group
No A or B genes
Group O
H gene
H substance
A or B genes
group A , B or AB
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Inheritance of ABO
• ABO & RH genes are not linked
• Genotype: Sum of the genes.
• Phenotypes: product of the genes.
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Allele from
the mother
Allele from
the father
Genotype of
offspring
Blood types of
offspring
A
A
AA
A
A
B
AB
AB
A
O
AO
A
B
A
AB
AB
B
B
BB
B
B
O
BO
B
O
O
OO
O
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Secretor Status
1. The presence or absence of the ABH antigens on the red
blood cell membrane is controlled by the H gene.
2. Presence or absence of the ABH antigens in secretions
is indirectly controlled by the Se genes.
3. In 80% of the population who possess secretor genes,
these antigens are also found in soluble form in secretions
and body fluids (e.g. plasma, saliva, semen and sweat).
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Think !!
• Is there any anti O Abs? why?
• A man with group O; married to a women
with group O. Is there any possibility that
they may have a group B child?
• Any group B person have B antigens in his
body fluids. Is this correct??
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