ABO Blood Group SystemABO Blood Group System

Importance Importance of ABO systemof ABO system

ABO compatibility between donor cell and patient serum is the essential foundation of pretransfusion testing

It is the only system with expected antibodies Whether they are IgG or IgM, ABO antibodies

can activate complement readily– This means that incompatibilities can cause life

threatening situations (transfusion reactions)

ABO antigensABO antigens

Biochemical & Genetic Biochemical & Genetic ConsiderationsConsiderations

ABO and H Antigen GeneticsABO and H Antigen Genetics

Genes at three separate loci control the occurrence and location of ABO antigens.

The presence or absence of the A, B, and H antigens is controlled by the H and ABO genes.

The presence or absence of the ABH antigens on the red blood cell membrane is controlled by the H gene.

The presence or absence of the ABH antigens in secretions is indirectly controlled by the Se gene.

ABO Antigen GeneticsABO Antigen Genetics

H gene – H and h alleles.

Se gene – Se and se alleles.

ABO genes – A, B and O alleles.

H AntigenH Antigen

The H gene codes for an enzyme that adds the sugar fucose to the terminal sugar of a precursor substance (PS)

The precursor substance (proteins and lipids) is formed on an oligosaccharide chain (the basic structure)

Type I and Type II PrecursorsType I and Type II Precursors

There are two potential precursors substances for ABH antigens Type I and Type II

Both are comprised of identical sugars but the linkage of the terminal sugars differs in the two types

Type I precursor has a terminal galactose linked to a subterminal N-acetylgluosamine in a 1-3 linkage

These same sugars combine in a 1-4 linkage in type II precursor

ABH Ags on red cells are derived from Type II chains whereas the ABH Ags in plasma are made from both types I & II precursors

Type II HType II H

After fucose is added to Type II chains, the structure is termed Type II H

Four kinds of Type II H have been identified

H1, H2 are simple straight chain glycolipidsWhereas H3 & H4 have branched chains

RBC Precursor StructureRBC Precursor Structure

Glucose

Galactose

N acetylglucosamine

Galactose

Precursor Substance (stays the

same)

RBC

Formation of the H antigenFormation of the H antigen

Glucose

Galactose

N-acetylglucosamine

Galactose

H antigen

RBC

Fucose

H antigenH antigen

The H antigen is the foundation upon which A and B antigens are built

A and B genes code for enzymes that add a sugar to the H antigen– Immunodominant sugars are present at the

terminal ends of the chains and confer the ABO antigen specificity

A and B AntigenA and B Antigen

The “A” gene codes for an enzyme (transferase) that adds N-acetylgalactosamine to the terminal sugar of the H antigen

– N-acetylgalactosaminyltransferase

The “B” gene codes for an enzyme that adds D-galactose to the terminal sugar of the H antigen

– D-galactosyltransferase

Formation of the A antigenFormation of the A antigen

Glucose

Galactose

N-acetylglucosamine

Galactose

RBC

Fucose N-acetylgalactosamine

Formation of the B antigenFormation of the B antigen

Glucose

Galactose

N-acetylglucosamine

Galactose

RBC

FucoseGalactose

GeneticsGenetics

The H antigen is found on the RBC when you have the Hh or HH genotype, but NOT from the hh genotype

The A antigen is found on the RBC when you have the Hh, HH, and A/A, A/O, or A/B genotypes

The B antigen is found on the RBC when you have the Hh, HH, and B/B, B/O, or A/B genotypes

H antigenH antigen

Certain blood types possess more H antigen than others:

O>A2>B>A2B>A1>A1B

Greatest amount of H

Least amount of H

The O alleleThe O allele

Why do Group O individuals have more H antigen than the other groups?

The O gene is a silent allele. It does not alter the structure of the H substance….that means more H antigen sites.

Group O Group A

Many H antigen sites

Most of the H antigen sites in a Group A individual have been

converted to the A antigen

Fewer H antigen

sites

A

A A

AA

Group O Group A

ABO SubgroupsABO Subgroups

ABO subgroups differ in the amount of antigen present on the red blood cell membrane– Subgroups have less antigen

Subgroups are the result of less effective enzymes. They are not as efficient in converting H antigens to

A or B antigens (fewer antigens are present on the RBC)

Subgroups of A are more common than subgroups of B

Subgroups of ASubgroups of A

The 2 principle subgroups of A are: A1 and A2

– Both react strongly with reagent anti-A

– 80% of group A or AB individuals are subgroup A1

– 20% are A2 and A2B

AA22 Phenotype Phenotype

Why is the A2 phenotype important?– A2 and A2B individuals may produce an anti-A1

– This may cause discrepancies when a crossmatch is done (incompatibility)

What’s the difference between the A1 and A2 antigen?– It’s quantitative– The A2 gene doesn’t convert the H3 & H4 to A very well

– The result is fewer A2 antigen sites compared to the many A1 antigen sites

AA11 and A and A22 Subgroups Subgroups

Anti-A antisera

Anti-A1 antisera

Anti-H lectin

ABO antibodies in

serum

#of antigen sites per

RBC

A14+4+0Anti-B900 x103

A24+03+Anti-B &

anti-A1

250 x103

Other A subgroupsOther A subgroups

There are other additional subgroups of A– Aint (intermediate), A3, Ax, Am, Aend, Ael, Abantu

A3 red cells cause mixed field agglutination when polyclonal anti-A or anti-A,B is used

Mixed field agglutination appears as small agglutinates with a background of unagglutinated RBCs

They may contain anti-A1

B SubgroupsB Subgroups

B subgroups occur less than A subgroups B subgroups are differentiated by the type of

reaction with anti-B, anti-A,B, and anti-H B3, Bx, Bm, and Bel

Other ABO conditionsOther ABO conditions

Bombay Phenotype (Oh) Inheritance of hh The h gene is an amorph and results in little or

no production of L-fucosyltransferase Originally found in Bombay (now Mumbai) Very rare

BombayBombay

The hh causes NO H antigen to be produced Results in RBCs with no H, A, or B antigen

(patient types as O) Bombay RBCs are NOT agglutinated with anti-A,

anti-B, or anti-H (no antigens present) Bombay serum has strong anti-A, anti-B and anti-

H, agglutinating ALL ABO blood groups What blood ABO blood group would you use to

transfuse this patient??

ANSWERANSWER::

Another Bombay– Group O RBCs cannot be given because they still have

the H antigen– You have to transfuse the patient with blood that

contains NO H antigen

ABO Blood GroupABO Blood Group

ABO AntibodiesABO Antibodies

ABO Blood Group SystemABO Blood Group System

The ABO Blood Group System was the first to be identified and is the most significant for transfusion practice

It is the ONLY system that the reciprocal antibodies are consistently and predictably present in the sera of people who have had no exposure to human red cells

Blood Group SystemsBlood Group Systems

Most blood group systems (ABO and others) are made up of:– An antigen on a red cell and the absence of it’s

corresponding antibody in the serum (if you’re A, you don’t have anti-A)

If you do NOT have a particular antigen on your red cells then it is possible (when exposed to foreign RBCs) to illicit an immune response that results in the production of the antibody specific for the missing antigen

ABOABO

Remember:– The ABO Blood Group System does NOT require the

presence of a foreign red blood cell for the production of ABO antibodies

– ABO antibodies are “non-red blood cell stimulated” probably from environmental exposure and are referred to as “expected antibodies”

– Titer of ABO Abs is often reduced in elderly and in patients with hypogammaglobulinemia

– Infants do not produce Abs until 3-6 months of age

ABO antibodiesABO antibodies

RBC PhenotypeFrequency(%) Serum Ab

A43Anti-B

B9Anti-A

AB4--------

O44 Anti-A,B

ABO antibodiesABO antibodies

IgM is the predominant antibody in Group A and Group B individuals– Anti-A– Anti-B

IgG (with some IgM) is the predominant antibody in Group O individuals– Anti-A,B (with some anti-A and anti-B)

ABO antibody factsABO antibody facts

Complement can be activated with ABO antibodies (mostly IgM, some IgG)

High titer: react strongly (4+)

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

Clinically Significant

Yes

Abs class

IgM, less IgG

Thermal range

4 - 37

HDNB

Yes

Transfusion Reactions

ExtravascularIntravascular

YesYes

ABO AntibodiesABO Antibodies

Usually present within the first 3-6 months of life Stable by ages 5-6 years Decline in older age & in hypogammaglobulinemia Newborns may passively acquire maternal antibodies

(IgG crosses placenta)

Nature of antibodiesNature of antibodies

Non-red blood cell stimulated – ABO antibodies

Red blood cell stimulated– Antibodies formed as a result of transfusion, etc– Usually IgG– Active at 37°C– Can occur in group O (may occur in group A or B)– These antibodies also occur in the other Blood

Group Systems

Anti-A,BAnti-A,B

Found in the serum of group O individuals Reacts with A, B, and AB cells Predominately IgG, with small portions being

IgM Anti-A,B is one antibody, it is not a mixture of

anti-A and anti-B antibodies

Anti-AAnti-A11

Group O and B individuals contain anti-A in their serum

However, the anti-A can be separated into different components: anti-A and anti-A1

Anti-A1 only agglutinates the A1 antigen, not the A2 antigen

There is no anti-A2.

Anti-A1

Clinically Significant

Sometimes

Abs class

IgM

Thermal range

4 - 22

HDNB

No

Transfusion Reactions

ExtravascularIntravascular

NoRare

Anti-HAnti-H

Auto-Anti-H

Clinically Significant

No

Abs class

IgM

Thermal range

4 - 15

HDNB

No

Transfusion Reactions

ExtravascularIntravascular

NoNo

Allo-Anti-H

Clinically Significant

Yes

Abs class

IgM, IgG

Thermal range

4 - 37

HDNB

Yes

Transfusion Reactions

ExtravascularIntravascular

YesYes

ABO Antigens in SecretionsABO Antigens in Secretions

Secretions include body fluids like plasma, saliva, synovial fluid, etc

Blood Group Substances are soluble antigens (A, B, and H) that can be found in the secretions.

This is controlled by the H and Se genes

Secretor StatusSecretor Status

The secretor gene consists of 2 alleles (Se and se) The Se gene is responsible for the expression of

the H antigen on glycoprotein structures located in body secretions

If the Se allele is inherited as SeSe or Sese, the person is called a “secretor”– 80% of the population are secretors

SecretorsSecretors

Secretors express soluble forms of the H antigen in secretions that can then be converted to A or B antigens (by the transferases)

Individuals who inherit the sese gene are called “nonsecretors”– The se allele is an amorph (nothing expressed)– sese individuals do not convert antigen precursors to H

antigen and has neither soluble H antigen nor soluble A or B antigens in body fluids

Secretor Status SummarySecretor Status Summary

The Se gene codes for the presence of the H antigen in secretions, therefore the presence of A and/or B antigens in the secretions is contingent on the inheritance of the Se gene and the H gene

Se gene (SeSe or Sese)

H antigen in secretions

A antigen

B antigen

se gene (sese)

No antigens secreted in saliva or other

body fluids

and/or

ABO GroupABH

Substances

Secretors (SeSe or Sese):ABH

A+++0+

B0++++

O00+++

AB+++++++

Non-secretors (sese):

A, B, O, and AB000

Sese + h/h (no H antigen) no antigens in secretions