ABO antigens are complex carbohydrates
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
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
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
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
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
[WG21:p540]
NB:
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"