BLOOD GROUPS IN MAN Flashcards
Antigen
Antigen: A substance that can stimulate an immune response.
An antigen collection
An antigen collection: A group of antigens that are phenotypically, biochemically or genetically related.
Antibody
Antibody: A blood protein produced in response to an antigen with the aim of counteracting that antigen.
Allele
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.
Anti-sera
Anti-sera: Commercially prepared blood group antibodies used in the laboratory for blood group testing.
Locus
Locus: A specific, fixed position on a chromosome where a particular gene is located (Plural: loci).
Genotype
Genotype: The genetic constitution of an individual.
Phenotype
Phenotype: The set of observable characteristics of an individual resulting from the interaction of its genotype with the environment.
What is a blood group?
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.
Discuss Antigens
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.
The clinically important blood group systems are (what are the 2 main)?
The clinically important blood group systems are:
ABO,
Rh (Rhesus),
Kell,
Duffy,
MN,
P,
Lewis,
Lutheran,
Kidd,
Li
Antibodies to blood group antigens could be produced as a result of…
Antibodies to blood group antigens could be produced as a result of:
- 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. - 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.
Characteristics of ABO blood group
Cold antibodies
These react optimally at 4°C (Range: 4°C - 25°C)
They are mostly IgM
Naturally occuring antibodies
Characteristics of Rh blood group
Warm antibodies
These react optimally at 37°C
They are mostly IgG
Antibodies formed due to environmental stimulant
History of the ABO blood group system
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
What is the protein that defines the ABO antigens?
glycosyl transferase
Discuss the features of AB,O BG
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.
Discuss the Bombay phenotype
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.
Discuss the variations in the ABO blood group system
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’.
ABO Test
Discuss The Rh blood group system
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.
What are the two variants of the D antigen?
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.
- Partial D:
Occurs when part(s) of the D antigen is (are) missing.
- Such individuals can produce antibodies (anti D) against the missing part.
Discuss the Rh antibodies
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
Clinical significance
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.
