A blood group system comprises the antigens, their corresponding antibodies and the genes coding for the antigens. Blood groups are inherited characters which give rise to the antigen-antibody system. Knowledge about blood groups is necessary for safe blood transfusion required in many cases. There are many blood group systems like Duffy, Kell, Lutheran, P, etc, but the most important of them all are the ABO and Rh system. It is because the antigens and antibodies of these systems are very powerful and give strong agglutination reactions in cases of wrong transfusions.
A Blood Group is made up of Antigens and their Corresponding Antibodies
The ABO blood group system comprises two antigens, A and B. Individuals possessing the A antigen on the surface of their red blood cells (RBCs) are said to have the A blood group. They also have anti-B antibodies in their serum. Individuals possessing the B antigen on the surface of their RBCs are said to have the B blood group. They have anti-A antibodies in their serum. O blood group individuals have neither A nor B antigen on their RBCs but they do possess anti-A and anti-B antibodies in their serum, while AB blood group individuals have both A and B antigens on their RBCs and no antibodies in their serum.
The H Substance
A and B antigens are basically glycoproteins. Each molecule is made up of a peptide backbone and sugars, from inside out; N-acetyl galactosamine, D-galactose, N-acetyl glucosamine and a terminal D-galactose. Blood group specificity is determined by the terminal sugar. To this precursor substance is added another sugar, L-fucose by the action of an enzyme fucosyl transferase. This enzyme is coded by a gene called H gene. The substance thus formed is called H substance.
The A and B Antigens
An A blood group individual possesses A gene which codes for an enzyme called N-acetyl galactosamine transferase. This enzyme transfers the sugar N-acetyl galactosamine to the terminal sugar on the H substance, thus forming the A substance or A antigen. A B blood group individual possesses B gene which codes for an enzyme called D-galactose transferase. This enzyme transfers the sugar D-galactose to the terminal sugar on the H substance, thus forming the B substance or B antigen.
In case of individuals having AB blood group, two different sugars, N-acetyl galactosamine and D-galactose, are transferred to different chains of the same RBC. It is worthwhile to note that the A and B antigens are formed only when terminal sugar L-fucose is added to the precursor substance. Thus, in the absence of the H substance, there is no production of A or B antigen, even though the corresponding genes are present.
References -
R. A. Goldsby, T. J. Kindt, and B. A. Osborne. 2000. Kuby Immunology, 4th edition, W. H. Freeman and Co., NY
R. Tizard. 1998. Immunology: An Introduction, 2nd edition, Saunders College Publishing
M. Roitt, J. Brostoff, D. K. Male. 1986. Immunology, Gower Medical Publishing Ltd.
Jitendra Rathod - Jitendra holds a Master's Degree in Microbiology. He had been a Lecturer, teaching the subject to undergrads and postgrads for 8 years.
Join the Conversation