ABO system

Basics

ABO antigens are complex carbohydrates

• Group A
  = A antigen with anti-B antibody
• Group B
  = B antigen with anti-A antibody
• Group AB
  = Both A antigen and B antige, with no antibodies
• Group O
  = Neither antigen (but still has H antigen) with anti-A and anti-B antibodies

Biochemical basis

H antigen is the core antigen
= a precursor with L-fucose as the terminal sugar
* On RBC, the precursor is mostly glycolipid
* In other tissues, the precursor is mostly glycoproteins

A antigen is formed when
* A terminal N-acetylgalactosamine is added to the H antigen

B antigen is formed when
* A terminal D-galactose is added to the H antigen

2 subgroups within group A

Both A1 and A2 agglutinate with anti-A antibody

Only group A1 agglutinate with lectin from Dolichos biflorus (a plant seed)
--> A1 = "Strong antigen"

The difference between A1 and A2 appears to be quantitative only

Genetic basis

ABO antigens are coded on
* 3 allelic genes A1, B1, and O1
* And 1 pair of allelic genes (H and h)

Genes for A and B are dominant
* e.g. Genotype AO and AA are both type A

Function of the genes

Gene H is responsible for enzyme alpha-L-fucosyltransferase
--> Attaches fucose to the precursor
--> Formation of the H antigen

Gene A is responsible for the enzyme alpha-N-acetyl-D-galactosaminyl transferase
--> Attaches a terminal N-acetylgalactosamine to the H antigen
--> Formation of A antigen

Gene B is responsible for the enzyme alpha-D-galactosyltransferase
--> Attaches a terminal D-galactose to H antigen
--> Formation of B antigen

Group O individuals has a single-base deletion in the corresponding gene
--> Open reading frame
--> Protein produced has no transferase activity

Location of ABO antigen

A1, B1, and H antigens are present in red cells and most other body cells
* e.g. white cells, platelets [PK1:p241]
* e.g. salivary glands, saliva, pancreas, kidney, liver, lungs, testes, semen, and amniotic fluid [WG21:p539]

In 80% of population,
--> A1, B1, and H antigens are also present in body fluids as soluble form
* e.g. plasma, saliva, semen, urine, gastric juice, tears, bile
* [PK1:p241]
* But NOT CSF

ABO antigens are stable and present in dried blood

Frequency in Caucasian population

[WG21:p540]

• Group A = 41%
• Group B = 10%
• Group AB = 4%
• Group O = 45%

NB:

• 80% of group A has strong antigen (A1)
• 20% of group A has weak antigen (A2)

ABO system antibodies

Antibodies in ABO system develop after 3 month of age
* Small amount of group A and group B antigen enter the body in bacteria and food
--> Stimulate formation of anti-A and/or anti-B antibodies
* [WG21:p540] Antigens very similar to A and B are common in intestinal bacteria and possibly food
--> Stimulate formation of anti-A and/or anti-B antibodies

Most anti-A and anti-B antibodies are IgM antibodies

Some people do develop IgG antibodies
--> The Australia Red Cross checks Group O blood for IgG antibodies and label them "Haemolysins present. Group O recipients only"