HEM 2133

Immunohaematology IImmunohaematology I

Lesson 4: Isoantibodies of ABO

System

Blood Group Antigens

Made of glycoprotein and glycolipid molecules

on surface of RBC

Normal part of RBC plasma membrane

E.g. Person with blood group A will have E.g. Person with blood group A will have

antigen A on RBC plasma membrane

Genetically determined (i.e. inherited from

mother and father)

Antigens of ABO system

Each individual inherits one ABO gene from

each parent

These two genes determine which Ags are

present on RBC membranepresent on RBC membrane

One position or locus on each chromosome

number nine is occupied by an A, B or an O

gene

A locus termed H and the final product of the

genes at that locus, H antigen, is necessary for

the expression of normal ABO antigens

The ABO genes do not code for the production

of ABO antigens, but rather produce specific

glycosyl transferases

Glycosyl transferases add sugars to a basic

precursor substance on the RBCs

The inheritance of at least one H gene (HH or The inheritance of at least one H gene (HH or

Hh) obtain the production of an enzyme called

-2-L-Fucosyl transferase, which transfers the

sugar to the terminal galactose of the

precursor chain

The H substance must be formed for the other

sugars to be attached in response to an

inherited A and/or B genes

ABO Genetics

Genes at three separate loci control the

occurrence and location of A and B antigens

1. Hh genes H and h alleles

H allele codes for a fucosyltransferase H allele codes for a fucosyltransferase

enzyme that adds a fucose on precursor

substance to form the H antigen onto which

A and B antigens are built on red blood cells

h allele is a silent allele (amorph)

2. Se genes Se and se alleles

Se allele codes for a fucosyltransferase

enzyme that adds fucose to precursor

substance in secretory glands

Controls expression of H antigens in secretions

(i.e. saliva, body fluids, etc)(i.e. saliva, body fluids, etc)

se allele is an amorph

Secretor gene controls only the presence or

absence of the H substance in body secretions

It does not affect the presence of the H

substance on erythrocytes

3. ABO genes A, B and O alleles

A and B alleles code for glycosyltransferase (a

fucosyltransferase enzyme that add a sugar

onto H antigens to produce A and B antigens)onto H antigens to produce A and B antigens)

O allele does not code a functional enzyme

Hh gene H and h alleles

(h is an amorph)

Controls presence of H, A

and B antigens on both

RBCs and in secretions

Se gene Se and se

alleles (se is an amorph)

Controls presence of H

antigen in the secretionsalleles (se is an amorph) antigen in the secretions

ABO genes A, B and O

alleles

Inherit 1 gene from each

parent that codes for an

enzyme that adds a sugar

to the H antigen

H Antigen

The H gene codes for an enzyme

(fucosyltransferase) that adds a fucose to the

terminal sugar of a precursor substance

H antigen is the foundation upon which A and H antigen is the foundation upon which A and

B antigens are built

A and B genes code for enzymes that add an

immunodominant sugar to the H antigen

Formation of the A Antigen

The A gene codes for an enzyme that adds

GalNAc (N-Acetyl-D-galactosamine) to the

terminal sugar of the H Antigen

Formation of the B Antigen

B gene codes for an enzyme that adds D-

Galactose to the terminal sugar of the H

antigen

The H antigen is found on the RBCs when

there is an Hh or HH genotypes but not with

the hh genotype

The A antigen is found on the RBCs when The A antigen is found on the RBCs when

there is Hh, HH and A/A, A/O or A/B

genotypes

The B antigen is found on the RBCs when

there is Hh, HH and B/B, B/O or A/B genotypes

Amount of H Antigen According to

Blood Group

Blood group O people have red blood cells rich

in H antigen

Neither the A or B genes have converted the H

antigens to A or B antigens just a whole antigens to A or B antigens just a whole

bunch of H

Amount of H antigen

O > A2 > B > A2B > A1 >A1B

Subgroups of ABO

Subgroups differ in the amount of antigen

expressed on the red cell membrane (e.g. A1

red cells express about 5 times more A antigen

than A2 red cells)

Some subgroups highly branched, complex Some subgroups highly branched, complex

antigenic structures

Some subgroups simplified linear antigenic

structures

Most common subgroups A

Subgroups of A

A1 and A2

20% of group A and AB person belong to

subgroup A2 and 80% to A1 (both types of red

cells react with anti-A)

A failure to classify a weak subgroup of A may A failure to classify a weak subgroup of A may

lead the donor to be classified as a group O

and to be transfused to a group O patient

All group O donors must be tested with anti-A,

B to confirm that they are not actually weak

subgroups of A

Other ABO Subgroups

The other weaker subgroups of A exist

(extremely rare) : A3, Aint, Am, Ax, Ael

Subgroups of B even rarer

Bombay Phenotype

Normal people inherit a pair of H genes at a

locus separate from the ABO locus

The classic Bombay phenotype would be

expected to arise most frequently in the expected to arise most frequently in the

children resulting from consanguineous

marriages

Results in inheritance of an h gene from both

parents at the H locus

Unable to produce the H glycosyltransferase

Unable to produce H antigen

Regardless of the A, B or O genes inherited by

the individual , no A, B or H antigen is formed the individual , no A, B or H antigen is formed

In a homozygous hh individual, even if ABO

genes are present, the precursor for the

synthesis of these antigens is not produced in

the absence of H gene

This rare blood group is called Bombay group

(Oh)

Typed as group O persons with anti-A and

anti-B sera

Only anti-H can detect the Bombay blood Only anti-H can detect the Bombay blood

group

(other groups will give a positive reaction with

anti-H serum, the Bombay group will give a

negative reaction)

Antibodies of ABO system

Anti-A

Arises in sera of group B people

Will agglutinate the RBCs of all group A and

AB peopleAB people

Most of the anti-A is IgM, although small

amount of IgG and IgA may be present

Anti-A can functionally split into anti-A1, which

react with A1cells but not with A2cells

A1 antigen is more branched and therefore

reacts differently with anti-A than the less-

branched A2 antigen

A reagent made from the plant Dolichos A reagent made from the plant Dolichos

biflorus (lectin) can easily differentiate

between A1 and A2 cells

Anti-B

Serum from group A people contains an

antibody that agglutinates essentially all

group B and group AB RBCsgroup B and group AB RBCs

This antibody also readily agglutinates cells

suspended in saline, activates complement

and may rapidly destroy incompatible RBCs by

intravascular hemolysis

Anti-A, B

Found in sera of all group O people

Used to confirm group O donors and aid in

identification of weak subgroups of A and B

Anti-H Anti-H

May be found as a weak, cold-reacting antibody

in the sera of group A1 and A1B people

Found in the sera of people expressing the

Bombay phenotype

When tested with anti-H or U. europaeus lectin,

the RBCs of the Bombay phenotype are negative

Universal Donor

Group O

The red blood cells lack both A and B antigens

Group O donor red cells can be used in times

of urgency or emergency release of donor of urgency or emergency release of donor

units

Universal Recipient

Group AB

Lacks circulating ABO antibodies in plasma

May receive transfusion of RBCs from any ABO

phenotypephenotype

Beware: other antibodies may be present