A elaborate note on "ABO Blood Group System" All the concept related to ABO blood group (i.e.; from basics to genetics, biochemistry & heredity) is clearly described here. If any question your mind approach, comment bellow, your doubt will be surely cleared through my reply. THANK YOU

1. ABO Blood Group System
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INTRODUCTION
There are total 365 types of classification of blood. For example ABO, Rh,
MNS, Lewis, Ss, P, Kell, Duffy, Kidd, Lutheran (Lu), li system etc. All of these systems
are categorised into following three.
 Major blood group system: ABO system & Rh system
 Minor blood group system: MNS system, P system etc..
 Familial blood group system (found in only few families): Kell
system, Duffy system, Kidd system etc.
Mixing of different blood type can cause blood clumping. Major blood group
system includes these blood classification system where different blood type transfer
is fatal. Transfer of blood types belonging to minor blood group system causes mild
blood clumping.
Before 1990 BC there was no concept of different blood types or blood
groups. Hence blood was transferred between any human. Even animal blood was also
used. Near about 1990 BC Karl Landsteiner observed the fatality during surgical blood
transfer. He performed some experiments in test tube and came on the decision, “The
serum of healthy human beings not only agglutinates animal red blood cells, but also
often those of human origin, from other individuals. It remains to be seen wheater
this appearance is related to inborn differences between individuals of it is the result
of some damage bacterial kind.” Thus he classified blood into four groups; i.e. A, B,
AB and O.
N.B. Agglutination is the blood clumping caused by mixing different blood
types.
AGGLUTINOGEN & AGGLUTININ
Antigen is a substance which is non-self to immune system and induces an
immune response in the body, especially the production of antibodies.
Agglutinin is an antigenic substance present in blood cells, bacteria, etc.,
which stimulates the formation of an antibody in blood serum.
Antigens function to enable our immune system to distinguish the body’s
own cells from foreign invaders. When invaders are recognised, they are destroyed by

2. antibodies, which are also produced by the immune system. In blood typing, antigens
on the surfaces of red blood cells (RBCs) are also known as agglutinogens and the
antibodies that react against them are also called agglutinins. Antibodies in the blood
are found in the plasma.
In easy words antigens related to blood grouping is termed as
agglutinogen and antibodies are termed as agglutinin.
BASIC CONCEPT OF ABO SYSTEM
All blood classification is based on the presence of membranous
agglutinogens on RBC bio membrane. In case of ABO system two types of
agglutinogens are considered; i.e. A agglutinogen and B agglutinogen.
People having only A agglutinogen on the surface of their RBC are belong to
blood group A and vice versa for blood group B. People possess both agglutinogens
are of blood group AB and possess none of these are of blood group O.
According to this concept Landstainer came on previously mentioned four
blood groups (i.e. A, B, AB & O).
CHEMICAL ASPECT
Either A or B both have a common portion in their chemical structure. That
common structure is H substance or H antigen or H agglutinogen (Figure 1). When H
agglutinogen binds with N-acetylgalactosamine or NAGA, it is agglutinogen A. When H
agglutinogen is bound with galactose, it is agglutinogen B.
Hence,
H agglutinogen + NAGA = A agglutinogen
H agglutinogen + Galactose = B agglutinogen
H antigen possess a monosaccharide, fucose with the terminal galactose
molecule.
Ceramide is RBC membranous anchors the H agglutinogen. Ceramide is not
the structural unit of H agglutinogen.

3. Figure 1: Structure of different antigens (agglutinogens) belonging to ABO system.
GENETICS OF ABO SYSTEM
During the formation of erythrocyte these A or B or both agglutinogens are
formed on its surface. Pre erythrocytic stage possess nucleus and corresponding
chromosome and genes. Through transcription and translation of specific genes A and
B agglutinogens forms. Procedure is describes bellow step by step.

4. Step 1
Plasma membrane of pre erythrocytic stage contains many oligosaccharide (usually
mentioned as oligosaccharide precursor; Figure 2) regarding to ABO system.
Larger arm (q) of 19th
chromosome contains a gene (i.e. H gene) that forms Fucose
Transferase. That enzyme attaches fucose monosaccharide to the terminal galactose
of oligosaccharide precursor. Thus H agglutinogen forms.
This is general step of all blood type.
Step 2
A agglutinogen formation: Thereafter a gene from larger arm (q) of 9th
chromosome forms an enzyme, transferase A, which attaches NAGA to the terminal
galactose of H agglutinogen. Thus A agglutinogen forms.
B agglutinogen formation: In this case there exists another gene in the same locus
which codes for transferase B, which transfers galactose at the terminal galactose of H
agglutinogen. Thus B agglutinogen forms.
HERIDITY OF ABO SYSTEM
Gene responsible for A agglutinogen formation at 9th
chromosome is iA
and
iB
for B agglutinogen. Inactive form of that gene is i. iA
, iB
and i are different alleles of
same gene of that particular locus. Inactive i gene does not codes any agglutinogen
(neither A nor B).
iA
or iB
both are dominant on i but iA
and iB
shows codominance over each
other.
For A blood group genotype may be iA
i (heterozygous) or iA
iA
(homozygous)
and vice versa for B blood group. For AB blood group there is always heterozygous
genotype i.e. iA
iB
due to codominance. For O blood group there is only homozygous
genotype i.e. ii.
Table 1: ABO Blood Group System
Genotype Agglutinogen Blood group
(Phenotype)
iA
iA
or iA
i A A
iB
iB
or iB
i B B
iA
iB
A & B AB
ii (nil) O

5. Figure 3: Example of heredity in ABO system. Here 1st generation is heterozygous A and B. There
are probability of all four types of blood group in next generation.
AGGLUTININ
In the blood plasma there also exists agglutinin of absent agglutinogen. For
example . . .
 Blood group A people have anti-B agglutinin in their blood plasma and vice versa
for blood group B.
 Blood group AB people possess none of these agglutinins (neither anti-A nor anti-
B).
 Blood group O people have both agglutinins as both agglutinogens are absent.

6. Table 2
Blood group Agglutinogen (on RBC) Agglutinin (at blood
plasma)
A A anti-B or β
B B anti-A or α
AB A & B (nil)
O (nil) α & β
DEVELOPMENTAL ASPECT
Agglutinins start to appear in the blood within a few months after birth, the
reason for their presence is not clear. Perhaps they are formed in response to bacteria
that normally inhabit the gastrointestinal tract. Because the antibodies are large IgM-
type antibodies that do not cross the placenta, ABO incompatibility between a mother
and her foetus rarely causes problems.
TRANSFER OF BLOOD
Transfer of blood from one body to another is medically termed as
transfusion.
Table 3

7. Transfusion definition: A transfusion is the transfer of whole blood or blood
components (red blood cells only or blood plasma only) into the bloodstream or
directly into the red bone marrow.
Blood transfusion between different blood type can cause blood clumping,
medically which is termed as agglutination. Agglutination through a series of reactions
causes haemolysis of RBC.
Agglutination definition: Agglutination is an agglutinogen–agglutin response in
which RBCs become cross-linked to one another. (N.B. agglutination is not the same
as blood clotting.)
Although transfusion between different blood type is fatal but AB can
receive from any blood type and O blood group can donate to any blood type. Logic
behind the universal concept is mentioned bellow.
Concept of universal recipient and universal donor:
Consider what happens if a person with type A blood receives a transfusion of type B
blood. The recipient’s blood (type A) contains A agglutinogens on the red blood cells
and anti-B agglutinins in the plasma. The donor’s blood (type B) contains B
agglutinogens and anti-A agglutinins. In this situation, two things can happen. First,
the anti-B agglutinins in the recipient’s plasma can bind to the B agglutinogens on the
donor’s erythrocytes, causing agglutination and haemolysis of the red blood cells.
Second, the anti-A agglutinins in the donor’s plasma can bind to the A agglutinogens

8. on the recipient’s red blood cells, a less serious reaction because the donor’s anti-A
agglutinins become so diluted in the recipient’s plasma that they do not cause
significant agglutination and haemolysis of the
recipient’s RBCs.
People with type AB blood do not have anti-A or
anti-B agglutinins in their blood plasma. They
are sometimes called universal recipients
because theoretically they can receive blood
from donors of all four blood types. They have
no agglutinins to attack agglutinogens on
donated RBCs.
People with type O blood have neither A nor B
agglutinogens on their RBCs and are sometimes
called universal donors because theoretically
they can donate blood to all four ABO blood
types. Type O persons requiring blood may receive only type O blood.
Practical aspect of universal recipient and donor concept:
In practice, use of the terms universal recipient and universal donor is misleading and
dangerous. There are two reasons behind this.
If injury is serious then much more blood maybe need during surgery, which alters
the previously mentioned dilution concept. Above this . . .
Blood contains antigens and antibodies other than those associated with the ABO
system that can cause transfusion problems. Thus, blood should be carefully cross-
matched or screened before transfusion.
STATISTICS OF ABO BLOOD GROUP (INDIA)
The NCBI study showed that O was the most common blood group
(37.12%) in the country closely followed by B at 32.26%, followed by A at 22.88%
while AB was the least prevalent group at 7.74%.

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