ABO Blood Group System

History: Karl Landsteiner

Discovered the ABO Blood Group System in 1901

He and his five co-workers began mixing each others red cells and serum together and inadvertently performed the first forward and reverse ABO groupings

Why is it important?

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 antigens:

Biochemical & Genetic Considerations

ABO 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 Genetics

H gene – H and h alleles (h is an amorph)

Se gene – Se and se alleles (se is an amorph)

ABO genes – A, B and O alleles

H 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)

RBC Precursor Structure

Glucose

Galactose

N-acetylglucosamine

Galactose

Precursor Substance (stays the

same)

RBC

Formation of the H antigen

Glucose

Galactose

N-acetylglucosamine

Galactose

H antigen

RBC

Fucose

H 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 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 antigen

Glucose

Galactose

N-acetylglucosamine

Galactose

RBC

FucoseN-acetylgalactosamine

Formation of the B antigen

Glucose

Galactose

N-acetylglucosamine

Galactose

RBC

FucoseGalactose

Genetics

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 antigen

Certain blood types possess more H antigen than others:

O>A2>B>A2B>A1>A1BGreatest

amount of HLeast

amount of H

The 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

Fewer H antigen

sites

A

A A

AA

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

converted to the A antigen

ABO 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 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

Secretors 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 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): A B H

A +++ 0 +

B 0 +++ +

O 0 0 +++

AB +++ +++ +

Non-secretors (sese):

A, B, O, and AB 0 0 0

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

Type 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

Type 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 glycolipids Whereas H3 & H4 have branched chains

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

ABO 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 A

The 2 principle subgroups of A are: A1 and A2

Both react strongly with reagent anti-A To distinguish A1 from A2 red cells, the lectin

Dolichos biflorus is used (anti-A1)

80% of group A or AB individuals are subgroup A1

20% are A2 and A2B

A2 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

A1 and A2 Subgroups

Anti-A antisera

Anti-A1 antisera

Anti-H lectin

ABO antibodies in serum

# of antigen sites per

RBC

A14+ 4+ 0 Anti-B 900 x103

A24+ 0 3+ Anti-B &

anti-A1

250 x103

Other 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 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 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

Bombay Phenotype

Bombay 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??

ANSWER:

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

Important Issue ‼

ABO Blood Group

ABO Antibodies

Landsteiner’s Rule:

Normal, Healthy individuals possess ABO antibodies to the ABO antigen absent from their RBCs

ABO 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 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

ABO 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 antibodies

RBC Phenotype

Frequency (%)

Serum Ab

A 43 Anti-B

B 9 Anti-A

AB 4 --------

O 44 Anti-A,B

Anti-A1

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

Extravascular Intravascular

No Rare

Anti-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

ABO 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 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

Extravascular Intravascular

Yes Yes

ABO 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 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-H

Auto-Anti-H

Clinically Significant

No

Abs class

IgM

Thermal range

4 - 15

HDNB

No

Transfusion Reactions

Extravascular Intravascular

No No

Allo-Anti-H

Clinically Significant

Yes

Abs class

IgM, IgG

Thermal range

4 - 37

HDNB

Yes

Transfusion Reactions

Extravascular Intravascular

Yes Yes