T2 - The ABO Blood Group

Question 1

ABO Blood Group and the Antigens they Possess

Answer 1

A group - A antigen
B group - B antigen
AB group - A and B antigen
O group - H antigen

Question 2

ABO Antigens

Answer 2

Not fully developed at birth
Carbohydrate (CHO) in nature
Formed from a common precursor substance
- type 1 and type 2
- chain of specific sugars
- type 2 is found only on RBC surface
H antigen is formed from common precursor substance
A and B antigens are formed from H antigen

Question 3

Formation of H, A and B Antigens

Answer 3

Produced by the addition of a single CHO residue to the precursor substance
- ‘immunodominant’ sugar/residue
- defines the antigen
Performed by enzymes called glycosyltransferases
Glycosyltransferases are encoded by different genes
- H, A, and B genes encode specific glycosyltransferases
- O gene is an amorph gene

Question 4

Antigens and the Glycosyltransferases they Code For

Answer 4

FUT1 (H)
- codes for a-2-L-fucosyltransferase
- adds L-fructose to terminal D-galactose of type 2 precursor substance
A gene
- codes for a-3-acetylgalactosaminyltransferase
- adds N-acetyl-D-galactosamine to terminal D-galactose of H antigen
B gene
- encodes for a-3-D-galactosyltransferase
- adds D-galactose to terminal D-galactose of H antigen

Question 5

ABO Antibodies

Answer 5

Produced against antigens that are not present on the RBC surface
A group - anti-B antibody in plasma
B group - anti-A antibody in plasma
AB group - none
O group - anti-A, anti-B, Anti-A,B antibodies in plasma
Naturally occurring
- produced without prior exposure to antigen on RBC
Usually IgM, but can also be IgG
Bind complement
React optimally at RT
Not detected until 3-6 months of age
Clinically are very important as can cause:
- intravascular haemolysis => death
- haemolytic disease of the newborn

Question 6

Forward and Reverse Grouping Reactions

Answer 6

Forward
- react patients RBC with known antiserum
- identifies which antigens are present on patients RBCs
Reverse
- react patients plasma with cells of known phenotype (A or B cells)
- identified which antibodies are present in patient plasma
Forward and reverse results much match

Question 7

Universal Donors and Universal Recipients

Answer 7

Group O people are ‘universal donors’
- can donate pRBCs to people of any ABO blood group
Group AB people are ‘universal recipients’
- can receive pRBCs of any ABO blood group

Question 8

ABO Subgroups

Answer 8

Arise from mutations in the glycosyltransferase genes
Relevant glycosyltransferase has diminished activity
Decreased antigen expression on RBC surface
Both Group A and Group B
Rarely occur but are important to identify