BLOOD GROUPS IN MAN

Question 1

Antigen

Answer 1

Antigen: A substance that can stimulate an immune response.

Question 2

An antigen collection

Answer 2

An antigen collection: A group of antigens that are phenotypically, biochemically or genetically related.

Question 3

Antibody

Answer 3

Antibody: A blood protein produced in response to an antigen with the aim of counteracting that antigen.

Question 4

Allele

Answer 4

Allele: An alternative form of a gene that arises by mutation, and is found at the same location on the chromosome as the original gene.

Question 5

Anti-sera

Answer 5

Anti-sera: Commercially prepared blood group antibodies used in the laboratory for blood group testing.

Question 6

Locus

Answer 6

Locus: A specific, fixed position on a chromosome where a particular gene is located (Plural: loci).

Question 7

Genotype

Answer 7

Genotype: The genetic constitution of an individual.

Question 8

Phenotype

Answer 8

Phenotype: The set of observable characteristics of an individual resulting from the interaction of its genotype with the environment.

Question 9

What is a blood group?

Answer 9

Blood group: A group of red blood cell antigens (Ags) coded for by alleles at a single gene locus or by linked loci.

Individuals who lack a red cell antigen can develop antibodies against such an antigen.

Such antibodies are either naturally occurring or immune antibodies.

Question 10

Discuss Antigens

Answer 10

The red cell membrane contains many anchored surface proteins.

Many of these surface proteins carry the different blood groups.

Most blood group antigens are glycoproteins.

Each blood group system represents either a single gene or a cluster of closely linked homologous genes.

About 400 blood group antigens have been described.

Thirty eight (38) are formally registered with the International Society for Blood Transfusion (ISBT).

These 38 are the formally recognized blood group systems.

Question 11

The clinically important blood group systems are (what are the 2 main)?

Answer 11

The clinically important blood group systems are:
ABO,
Rh (Rhesus),
Kell,
Duffy,
MN,
P,
Lewis,
Lutheran,
Kidd,
Li

Question 12

Antibodies to blood group antigens could be produced as a result of…

Answer 12

Antibodies to blood group antigens could be produced as a result of:

1. Response to environmental antigens:
   These are termed as “naturally occurring antibodies”.
   They are produced as a result of exposure to substances like food and bacteria which have chemical structures similar to some blood antigens.
   This is typical of the ABO blood group system.
2. Response to foreign blood group antigen:
   - These are termed as “immune antibodies”
   - Result from exposure by transfusion, pregnancy and transplantation.
   - This is typical of the Rh blood group system.

Question 12

Characteristics of ABO blood group

Answer 12

Cold antibodies
These react optimally at 4°C (Range: 4°C - 25°C)
They are mostly IgM
Naturally occuring antibodies

Question 12

Characteristics of Rh blood group

Answer 12

Warm antibodies
These react optimally at 37°C
They are mostly IgG
Antibodies formed due to environmental stimulant

Question 13

History of the ABO blood group system

Answer 13

It was first discovered in 1901 by Karl Landsteiner.

The ABO gene resides on chromosome 9.
The most important blood group in transfusion and transplantation.

Not fully developed at birth; develops by the sixth month of life.

The antibodies act optimally at 4°C and are mostly IgM and thus referred to as cold antibodies

Question 14

What is the protein that defines the ABO antigens?

Answer 14

glycosyl transferase

Question 15

Discuss the features of AB,O BG

Answer 15

It is encoded from a single gene from which there are three major alleles: A, B and O.

Biochemically, the A, B and O blood groups have a common component called the ‘H substance’(or H antigen).

This H substance is a basic antigenic glycoprotein with a terminal sugar (L-fucose) on the RBC

The A and B alleles catalyze the addition of different carbohydrate residues to this H substance.

The A allele catalyzes the addition of N-acetyl galactosamine to the H substance, leading to the formation of the A antigen (Blood group A).

The B allele catalyzes the addition of d-galactose to the H substance, leading to the formation of the B antigen (Blood group B).

The O allele is non-functional so it does not modify the H substance. It leaves it as it is, leading to the formation of the O antigen (Blood group O). Because the H substance is not modified, the O antigen is sometimes referred to as the H antigen.

Question 16

Discuss the Bombay phenotype

Answer 16

There is a rare phenomenon seen in some individuals; they lack the H substance.
These individuals are neither blood group A, B, AB or O.
They are referred to as having the Bombay phenotype
This was first discovered in Bombay (now Mumbai), India in 1952.

Question 17

Discuss the variations in the ABO blood group system

Answer 17

The A allele has two variants: A1 and A2.

These are however of minor clinical significance

Majority of human blood can be grouped into 6 main ABO phenotypes.

They are O, A₁, A₂, B, A₁B and A₂B.

The A, B and H antigens are present on most body cells including WBCs and platelets

About 80% of the human population possess water-soluble ABO blood group antigens in their body fluids (plasma, saliva, semen, sweat, etc). These individuals are called ‘secretors’. The 20% that do not possess this ability are called ‘non-secretors’.

Question 18

ABO Test

Question 19

Discuss The Rh blood group system

Answer 19

This system was the 4th to be discovered.
It’s however the second most important in blood transfusion.
The gene encoding the Rh blood group is on chromosome 1.

Thus, there are five major antigens in the Rh system: D, C, c, E and e.
The D antigen is the most clinically significant among the five.
About 96% of Nigerians are Rh D positive.

Question 20

What are the two variants of the D antigen?

Answer 20

There are two variants of the D antigen called weak D and Partial D.

Weak D:
- Occurs as a result of reduction in the number of D antigen sites on the red cells.
It was previously referred to as Du cells.
They react weakly with anti D sera during blood grouping testing and can be confused as Rh negative.

2. Partial D:
   Occurs when part(s) of the D antigen is (are) missing.
   - Such individuals can produce antibodies (anti D) against the missing part.

Question 21

Discuss the Rh antibodies

Answer 21

Rh antibodies are usually immune antibodies; they develop when an individual who is Rh D negative is exposed to Rh D +ve blood (e.g. from blood transfusion or pregnancy).
The Rh antibodies are normally IgG and are warm antibodies.
The Rh antibodies include:
Anti-D
Anti-C
Anti-c
Anti-E
Anti-e

Question 22

Clinical significance

Answer 22

The ABO and Rh blood grouping test in the laboratory is based on an agglutination reaction. When RBCs carrying an antigen(s) are exposed to the corresponding antibodies (anti-sera), they interact with each other thereby forming visible agglutination or clumping.

Individuals who do not have a particular blood group antigen can produce antibodies to the absent blood group. This can lead to haemolytic blood transfusion reaction.

Rh antibodies (anti-D) are IgG antibodies capable of transplacental passage from mother to fetus. Thus, they are an important cause of haemolytic disease of the newborn.

A2 cells react weakly with anti-sera compared to A1 cells. Thus, individuals with A2B may be wrongly typed as B.

Individuals with the Bombay phenotype can donate RBCs to any member of the ABO blood group system BUT they cannot receive blood from any member of the ABO blood group system. In other words, they can only receive from themselves.

Blood from donors with weak D should never be transfused to Rh negative recipient because it can cause immuno-sensitization or blood transfusion reaction.

Patients with partial D should only be transfused with Rh negative blood.

For individuals with the Duffy blood group, the Duffy antigen serves as a receptor for Plasmodium vivax malaria parasites.