ANTIGEN SYSTEM

• Antigens defined as substances recognized by the body as foreign  and it cause the body to produce an antibody to react specifically with it
• In blood group system, the antigen system is ABO antigens
• The characteristic of ABO antigens are:

1. Glycolipid in nature – they are oligosaccharides attached   directly to lipids on red cell membrane
2. Stick out from red cell membrane and there are  many antigen sites per red blood cell (approximately 800,000)
3. soluble antigens are present in RBC, plasma, saliva, and other secretions
4. ABO antigens are only moderately well developed at birth

• The antigen system consists of 3 types of antigen:

          1. A antigens

          2. B antigens

          3. H antigens

• The production of A, B and H antigens are controlled by the action of transferases
• Transferases are enzymes that catalyze (or control) addition of specific sugars to the oligosaccharide chain.
• The H, A, or B genes each produce a different transferase – adds a different specific sugar to the oligosaccharide chain.
• Also, there are 2 types of precusor chain of sugars is formed which either Type 1 or Type 2 depending on the linkage site between the N-acetylglucosamine (G1cNAc) and Galactose (Gal)

•H gene causes L-fucose to be added to the terminal sugar of precursor chain, producing H antigen. In O group, the H antigen remaining unchanged

•A gene causes N-acetyl-galactosamine to be added to H substance, producing A antigen

•B gene causes D-galactose to be added to H substance, producing B antigen

Diagram showing the carbohydrate chains that determine the ABO blood group

Diagram showing the types of blood tag on antibody and antigen present

• Group A have antigen A plus antigen H
• Group B have antigen B plus antigen H
• Group AB have both antigen A and antigen B plus antigen H
• Group O only have antigen H