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ABO Grouping

Overview

The ABO system is regarded as the most important blood-group system in transfusion medicine because of severe hemolytic transfusion reactions and, to a lesser degree, hemolytic disease of the newborn.

ABO grouping is a test performed to determine an individual’s blood type. It is based on the premise that individuals have antigens on their red blood cells (RBCs) that correspond to the four main blood groups: A, B, O, and AB. Antibodies (isohemagglutinins) in an individual’s plasma are directed against blood group antigens that their own RBCs lack (see Table 1, below). These antibodies (isohemagglutinins) form early in life. ABO antigens are expressed on RBCs, platelets, and endothelial cells and are present in body fluids.

ABO testing is performed to prevent an adverse transfusion reaction that could be caused by ABO incompatibility between the blood of a patient (recipient) and that of a donor.

Table 1. ABO Genotyping (Open Table in a new window)

Blood Group

Antigens Present on RBCs

Antibody Present in Serum

Genotype

A

A antigen

Anti-B

AA or AO

B

B antigen

Anti-A

BB or BO

AB

A antigen

B antigen

None

AB

O

None

Anti-A, anti-B, anti-A,B

OO

RBCs = red blood cells.

The gene FUT1, located on chromosome 19q13.3, is responsible for the synthesis of AB and H antigens.
Chromosome 9q34 encodes for A and/or B glycosyltransferases. These are the different transferases necessary to produce the various ABO antigens (mainly glycolipoproteins) on blood components. A separate “secretor”/”se” gene (FUT2) is also located on chromosome 19q13.3; this gene encodes for the transferases necessary to produce ABO antigens (mainly glycoproteins) that are affiliated with bodily fluids other than blood (eg, saliva). Most of the population (approximately 80%) expresses the secretor gene.

The precursor glycoprotein/glycolipoprotein that allows expression of all ABO antigens is composed of oligosaccharide chains with the essential addition of fructose. Alpha-2-L-fucosyltransferase is the enzyme responsible for adding fructose to the primary galactose of the oligosaccharide chain. This is the foundation “H” antigen. The gene for type “O” is silent and thus maintains the original formation of the H antigen.

For the configuration of A, B, and AB antigens, an additional alpha 1,3-N acetylgalactosaminyltransferase and/or glycosyltransferase enzyme is encoded to attach additional sugars to the “H” antigen. For type A, an N-acetyl-galactosamine is attached to the primary, initial galactose. For type B, an additional galactose is attached to the primary galactose. For type AB, both of these sugars would be added (see Table 2, further below).

ABO antigens and enzymes table; G1cNAC = N-acetlyg

ABO antigens and enzymes table; G1cNAC = N-acetlygalactosamine. Image created by Jaye Parsley.

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The O allele is an autosomal-recessive trait, and the A and B alleles are codominant traits. Each parent contributes an A, B, or O allele to their offspring, depending on the ABO type of the parents (see Table 1, above).

The frequency of ABO blood group antigens varies in different populations. Most people have the antigen “H” encoded, because this is the precursor for antigen A or B. Thus, depending on whether the other genes are encoded, it will then be determined if they will remain a type O or change to type A, B, or AB. Type O blood is the most frequent, and type B and AB are the least frequent (see Table 2, below).

Table 2. ABO Phenotype Frequencies Among Different Ethnic Groups (Open Table in a new window)

Race

O

A

B

AB

White

44%

43%

9%

4%

Black

49%

27%

20%

4%

Asian

43%

27%

25%

5%

 Source: Adapted from several sources.

In rare cases, even the initial precursor H antigen is not genetically encoded. These individuals are known as Bombay and are able to receive only Bombay type blood, because they make antibodies to not only type A, B, or AB donor RBCs but also to type O donor red cells (anti-H), causing hemolysis of the transfused donor RBCs.

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