ABO and Rh Blood Grouping Systems

Question 1

How is blood grouped?

Answer 1

Blood can be grouped according to the expression of antigens on the surface of the RBCs

Carbohydrate-based systems

- ABO
- Lewis
- P
- li

Protein-based systems:

- Rh
- Kidd
- Kell
- Duffy
- Lutheran
- MNS

Question 2

What are carbohydrate antigens?

Answer 2

The carbohydrate antigens are immunodominant sugars which are attached to a precursor molecule by action of a glycosyl transferase enzyme

It is these enzymes that are the products of the blood group genes
(( gene codes for the enzyme that makes the antigen that determines your blood group, NOT THE ANTIGEN ITSELF ))

Question 3

Which antibodies are present for each blood group?

Answer 3

Blood Group Antigens Present Antibodies Present
A A Anti-B
B B Anti-A
AB Both A and B Neither Anti-A nor Anti-B
O Neither A nor B Anti-A and Anti-B

In the case of the ABO system there are three possible alleles an individual can inherit: 
A allele (co-dominant)
B allele (co-dominant)
O allele (recessive)

So can be heterozygous A and B (with an O) or homozygous

Question 4

What do the A, B and O alleles code for?

Answer 4

The (ABO) alleles DO NOT code for the (ABO) antigen!!
– The allele codes for an enzyme that allows attachment of
the antigen

Allele: Enzyme: Function:
Allows attachment of
A N-acetyl-galactosaminyl N-acetyl-galactosamine
transferase to the H antigen

B D-galactosyl Allows attachment of
transferase D-galactose to the
H antigen

Question 5

What is the H antigen?

Answer 5

The H antigen is another carbohydrate structure = L-fucose

H antigen is bound to precursor chains on the red cell surface

Without the H-antigen, A and B carbohydrates cannot attach to the RBC membrane

The H antigen is present in ALL ABO blood types* A, B, AB and O

This H antigen is a prerequisite for transfer of A and B antigens on to the precursor chain

If A or B antigens are not attached to the H antigen, the individual is blood group O
* except in individuals with Bombay phenotype*

Question 6

How do H antigens attach to the surface of the RBCs?

Answer 6

Attachment of the H antigens to the precursor chains also requires enzyme activity

Two genes are involved in the production of these enzymes:

Gene: Enzyme:
FUT 1 α1,2-fucosyl transferase I
FUT 2 α1,2-fucosyl transferase II

Enzyme: Function: Product: Location:
α1,2-fucosyl Adds L-Fuc to Type 2 H Membranes
transferase I TYPE 2 chains antigens of RBC

α1,2-fucosyl Adds L-Fuc to Type I H Secretions
transferase II TYPE 1 chains antigens

Question 7

What is the difference between Type I and Type II precursor chains?

Answer 7

Type I:
– Primarily glycoproteins
– Found in secretions
– Galactose is bound to N-acetylglucosamine by a β1-3 linkage

Type II:
– Primarily glycolipids
– Found on cell surface
– Galactose is bound to N-acetylglucosamine by a β1-4 linkage

Question 8

How does A attach to the H antigen?

Answer 8

N-acetylgalactosaminyl transferase (Enzyme) attaches N-acetylgalactosamine (sugar) to the H antigen at terminal galactose at the 5 position, on Type I and Type II chains!

Question 9

How does B attach to the H antigen?

Answer 9

D-galactosyl transferase(Enzyme) attaches D-galactose (sugar) to the H antigen at terminal galactose at the 5 position, on Type I and Type II chains!

• Sugar:

Question 10

What is the Bombay phenotype?

Answer 10

Mutations in FUT1 result in lack of α1,2-fucosyl transferase I activity = no L-Fucose added and no H antigen expression

Even if A and B enzymes are synthesised – A and B antigen cannot be expressed

Blood from individuals with FUT1 mutation appears as
blood group O due to their lack of A and B

HOWEVER.. Their serum contains anti-A, anti-B AND anti-H (0.0004% of the population)

In transfusions – bombay individuals can only receive bombay blood

Question 11

What are the A antigen variants?

Answer 11

Individuals of blood group A can be further sub-grouped:
A1, A2, A3, Ax, Ael

> 99% of individuals classed as group A are A1 or A2

A1 and A2 genes code for slightly different A transferases

A2 transferase is less efficient at attaching A antigen (N- acetlygalactosamine) – RBCs have 20-25% less A antigen than A1 cells

The more A (and B) antigen that is made, the less H antigen is ‘seen’
– Presence of A (and B) antigen hides presence of H

Question 12

What is the Rh system?

Answer 12

Complex set of RBC surface PROTEINS

Presumed to have a role in maintaining the integrity of the cell membrane

Comprise approx. 50 antigens – largest blood group system

Mostly concerned with just 5 antigens D, C, c, E, and e -account for the vast majority of clinical and laboratory issues

Rh proteins are:
- Transmembrane proteins critical to the structure of the RBC membrane
- Rh null erythrocytes (lack all Rh proteins) are stomatocytic &
spherocytic – leads to haemolytic anemia
- Rh proteins mediate key interactions with the underlying cytoskeleton
- Also have important roles in ammonia transport

Question 13

What is the importance of RhD compatibility?

Answer 13

Important that RhD- individuals are not transfused with RhD+ blood

This is especially true in women of child bearing age

Transfusion of RhD+ blood cells results in the recipient’s immune system ‘recognising’ the RhD+ cells as foreign and producing large amounts of anti-D

RhD+ individuals can receive either RhD+ or RhD- blood
in a transfusion as their immune system will not respond to RhD antigen

Not quite as severe as ABO incompatibility, just depends on the amount of antigens seen by the body and reacted to

Question 14

What is the nature of Rh antigens?

Answer 14

Highly immunogenic - Exposure to <1ml Rh+ RBCs can stimulate antibody production in a Rh- individual

To date no ‘d’ antigen has been found - considered an amorph (silent allele)

        D                     c                     E                     C                      e Highly Immunogenic                                              Rarely Immunogenic

Question 15

What are the different Rh nomenclatures?

Answer 15

Fisher-Race: The DCE Terminology

Wiener (Rh-Hr): The Rh-Hr Terminology

Rosenfield: Alpha/Numeric Terminology

ISBT (International Society of Blood Transfusion): Numeric Terminology

Question 16

What was the Fisher-Race: The DCE Terminology?

Answer 16

The 5 main antigens were inherited as a combination of three 
closely linked genes: 
- one D gene
- one C/c gene
- one E/e gene

Each child would receive a group of Rh genes (HAPLOTYPE) from each parent

Combination of genes inherited from both parents results in the child’s final Rh phenotype

Rh genes are co-dominant - each inherited allele expresses its corresponding antigen on the RBC

Individual’s Rh phenotype is reported as ‘DCE’ - Fisher-Race postulated that the C/c locus lies between D/d and E/e loci

Rh D+ Haplotypes: Rh D- Haplotypes:
Dce dCe
DcE dcE
DCe dce
DCE dCE

This has since been proven wrong

Question 17

What was the Weiner (Rh-Hr) Nomenclature?

Answer 17

Weiner also believed there were three genes controlling DCE
expression

  RhD+                                 RhD- R1      DCe                           r’        dCe  R2      DcE                           r’’       dcE R0     Dce                            r         dce Rz      DCE                          r^y     dCE

((To learn: + means D is always uppercase, - means d is always lower case. The number/‘ signifies which (of c and e) are capitalised). r and y have both capitalised))

Question 18

What are the genetics of Rh?

Answer 18

Research has now proven that 2 closely linked genes control expression of ALL Rh antigens: Another example of Co-Dominance

RhD – controls expression of D antigen

RhCE – controls expression of C, c, E & e antigens

Both found on chromosome 1
- alternative splicing determines which you express

Question 19

What are the inherited RhD possibilities?

Answer 19

RhD possibilities:

• Two copies – RhD+
• One Copy – RhD+
• No copies – RhD-
• Abnormal copies – RhD variants or RhD-

Important to understand that there are over 70 RhD gene variants

Inheritance of one abnormal and one normal copy USUALLY results in a RhD+ phenotype

Can inherit one abnormal copy and one RHD deletion OR even two different RHD variant genes = RhD-

Question 20

What is weak RhD?

Answer 20

Caused by mutated forms of the RhD gene

Generally point mutations – single nucleotides

Lead to amino acid changes within the membrane or the intracellular parts of RhD
- Gene gives a transmembrane protein that constantly moves inter and
extra cellular.
- If a mutation occurs intracellularly it leads to decreased
(measurable) amount outside.
- In testing they could therefore appear as D-ve but they’re not, they’re
weak D+ve! Only bad if they are a donor

Leads to decreased amount of RhD present at the RBC surface for detection by anti-RhD

Weak RhD - Correct number of epitopes on RhD antigen, but decreased number of RhD molecules on RBC.

These patients are really RhD+ and so do not make anti-RhD antibodies

Therefore donating RhD+ blood to weak RhD recipients does not
cause a transfusion reaction