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ABO blood group SystemABO blood group System

By : Mr. Waggas Ela’as, M.Sc; MLTBy : Mr. Waggas Ela’as, M.Sc; MLT

• Discovered by Karl LandsteinerKarl Landsteiner; locus on chromosome 9

• Single most important blood group for the selection and transfusion of blood

• Widely expressed tissues & body fluids including red cells, platelets & endothelial cells

• Three antigens: A, B, H

• Two major antibodies: anti-A and anti-B

• Four phenotypes: A, B, AB, O A & B Ag’s autosomal co-dominant (expressed on group A, B and AB red cells; O phenotype autosomal recessive (most frequent)

ABO BLOOD GROUP SYSTEM

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People inherit H gene which will convert a “precursor Substance” into an H Substance. If the person –then – inherit A gene ; it will convert the H substance into A antigen. If he inherit B gene ; it will convert the H substance into B antigen. If he inherit both A & B genes ; they will convert the H substance into AB antigens. If he inherit NO A nor B genes; the H substance will remain unconverted. This is named “group O”.If the person didn’t inherit the H gene; he will not have an H substance. This is named “Bombay group”.

The A and B genesgenes control the synthesis of specific enzymes

(glycosyl-transferase) responsible for the addition of single

carbohydrate residues (N-acetyl galactosamine for group A

and D-galactose for group B) to a basic antigenic glycoprotein

or glycolipid with a terminal sugar L-fucose on the red cell,

known as the H substance. The O gene is an amorph and

does not transform the H substance.

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The H gene is converted to H substance. Subsequently the H

substance is acted upon by specific transferases which is

converted to either A or B antigen.

Some H substance remains unconverted and is expressed

as H antigen. Since there is no conversion of H substance

in O blood group, the amount of H antigen is maximum on

red cells (The O gene is a silent allele).

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ABO AntigensABO Antigens

• Present on the surface of red cells as well as tissue and endothelial cells in the body (found on all the cells of the body tissues, hence the ABO compatibility is a prerequisite in cases of organ transplants).

• Found in soluble form in plasma & other body secretions in people known as secretors.secretors.

• Inherited in simple Mendelian fashion from an individual’s parents.

• 3 possible genes that can be inherited: A, B, O

• A and B genes produce a detectable product

• O gene does not produce a detectable product

ABO BLOOD GROUP SYSTEM

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ABO BLOOD GROUP SYSTEM

Phenotype Antigen Natural antibody

Genotype

A A only Anti-B AA or AO

B B only Anti-A BB or BO

AB A and B None AB

O None Anti-A,

Anti-B

OO

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ABO BLOOD GROUP SYSTEM

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ABO BLOOD GROUP SYSTEM

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ABO BLOOD GROUP SYSTEM

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ABO BLOOD GROUP SYSTEM

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H Antigen

• required to produce either A or B antigens

• possible genetic combinations: HH, Hh, or hh

• HH or Hh (+) produce H Ag 99.99% of Caucasians

• hh does not produce H Ag Bombay phenotype (Oh)

• anti-H antibodies rare – found only in individuals with Bombay phenotype

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Example of determining offspring blood types from known or suspected genotypes:

Genotype parent #1 (AO)

A O

Genotype parent A AA AO

#2 (AB) B AB BO

Phenotypes of possible offsprings: A, AB, B

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Frequencies of ABO Blood Groups:

Blood Group Frequency

O 45%

A 41%

B 10%

AB 4%

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ABO Subtypes:[Subgroups of A are of minor clinical significance].

1. A variants (A1, A2)

• A1 most common (80%) & most antigenic

• A1 and A2 differentiated using antisera specific for A1 Ag (anti-A1 lectin) prepared from seed known as Dolichos biflorus (+) reaction with A1 but not A2

• Anti-A reacts with both A1 & A2 but more strongly with A2

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• A2 red cells have fewer A antigen sites than A1 cells. In

addition, the number of A antigens on A1 RBCs is

approximately five times more than on A2 RBCs.

• The H antigen content of red cells depends on the ABO

group and when assessed by agglutination reactions with

anti-H, The strength of reaction tends to be graded O > A2 >

A2B > B > A1 > A1B.

• Other subgroups of A are occasionally found (e.g., A3, Ax)

that result from mutant forms.

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ABO Subtypes:

2. Weak A and weak B phenotypes

3. Null phenotypes:

(a) Bombay (Oh)

• No A, B or H Ag on red cells & secretions

• With anti-A, anti-B & anti-H in their sera

(b) para-Bombay

• Absent or only trace A,B & H Ag’s detected on RBCs with normal expression in secretions & body fluids

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ABO Antibodies

• Natural antibodies antigenic stimulus is environmental exposure occurs from birth

• Newborns without ABO antibodies of their own; begin to produce Abs with detectable titer at 6 months of age

• Other characteristics of ABO antibodies:

1. IgM :(complete Abs).

2. Reacts at room temp. after an immediate spin (cold Abs : react best at 4oC, but can react also at 37oC).

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Summary Precursor substance

No H gene H gene

Bombay group H substance

No A or B genes A or B genes

Group O group A , B or AB

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Inheritance of ABO

• ABO & RH genes are not linked

• Genotype: Sum of the genes.

• Phenotypes: product of the genes.

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Allele from the mother

Allele from the father

Genotype ofoffspring

Blood types of

offspring

A A AA A

A B AB AB

A O AO A

B A AB AB

B B BB B

B O BO B

O O OO O

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Secretor Status1. The presence or absence of the ABH antigens on the red blood cell membrane is controlled by the H gene.

2. Presence or absence of the ABH antigens in secretions is indirectly controlled by the SeSe genes.

3. In 80% of the population who possess secretor genes, these antigens are also found in soluble form soluble form in secretions and body fluids (e.g. plasma, saliva, semen and sweat).

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Think !!

• Is there any anti O Abs? why?

• A man with group O; married to a women with group O. Is there any possibility that they may have a group B child?

• Any group B person have B antigens in his body fluids. Is this correct??

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