Blood group antigens are carbohydrates that attached to lipids or proteins. An antigen is a foreign substance to the body that induces an immune reaction. An immune response occurs when antibodies, proteins in immune systems, are summoned to attack an antigen. Creative Biolabs is known as a world-class institute and professional biotechnique company that has spent a large amount of money and energy in carbohydrate antigens, especially blood group antigens, to further explore their properties, structures, synthesis, and expressions in physical progressions. With the identification and analysis of these antigens, we can rapidly refresh the mechanism of their biological functions and extensively open insights into their practical potency.
The antigens expressed on the red blood cell (RBC) determine an individual's blood group. The two main blood groups are termed ABO (with blood types A, B, AB, and O) and Rh (with Rh D-positive or Rh D-negative blood types). Blood group antigens are either sugars or proteins, for instance, the antigen of ABO blood groups are sugars, while the antigen of Rh blood groups are proteins. These antigens are attached to various components in the RBC membrane.
The figure below exhibits the RBC membrane and some of the blood group antigens linked to it. Aside from the sugar (glycans or carbohydrates) antigens, the RBC membrane includes three types of protein: single-pass proteins, multi-pass proteins, and glycosylphosphatidylinositol (GPI)-linked proteins, that carry blood group antigens.
Fig.1 A model for the types of membrane components carrying blood group antigens. (Dean, 2005)
Usually, blood group antigens are surface markers on the outside of RBC membranes. Nevertheless, the blood group antigens are not only restricted to red cells and even to hematopoietic tissues. The antigens of ABO systems are widely distributed throughout the tissues and have been identified on the platelets and white cells as well as in the skin, kidney, urinary tract, the epithelial cell of the gastrointestinal (GI) tract, and the lining of blood vessels.
Blood group antigens are found on N-glycoproteins, O-glycoproteins, and glycolipids, both on RBCs and many other cells of the body. They’re synthesized on type 1, 2, 3, or 4 structures. Type 1 and 2 structures are Galβ1-3GlcNAc-R and Galβ1-4GlcNAc-R, respectively. Both are present on N- and O-glycoproteins, as well as on glycolipids. Type 2 structures are ubiquitous, while type 1 structures are found solely in the GI tract. Type 3 and 4 structures are all Galβ1-3GalNAc-R, however, the R group for types 3 and 4 differs. R for type 3 is serine (Ser) or threonine (Thr) of an O-glycopeptide, and R for type 4 is normally a glycolipid moiety.
Fig.2 Blood group antigens. (A) Glycan structure of blood group
antigens. (Cabezas-Cruz, 2017); (B) Structures of
human blood group A, B, and H. (Wang, 2006)
Synthesis of blood group antigens needs at least two steps. The first is the synthesis of H antigen, the structure corresponding to O blood type. The second is the synthesis of either A or B structure. The H antigen is formed by the addition of fucose in α1,2 linkage to a terminal galactose on a type 1-4 chain. After synthesis of the H structure, the A and B transferases, which differ by four amino acids, utilize the H structure to synthesize A and B antigens on type 1-4 chains. Two genetic loci encode the H transferase. The H loci is functional in RBCs and the secretor loci is functional in GI epithelial cells, obtaining its name for the blood group antigens produced by secreted glycoconjugates. These transferases are also essential in forming several Lewis antigens.
I. Transfusion reactions
Blood group antigens play a role in recognizing exotic cells in the bloodstream. For example, if one person with blood type A receives a blood transfusion with type B, the recipient's immune system will recognize the type B cells as foreign and induce an immune response. Antibodies against blood type B cells (anti-B antibodies) are made to attack and destroy the type B cells. The routine practice of blood typing and cross-matching blood products should prevent adverse transfusion reactions caused by ABO antibodies. This sort of blood type mismatch will lead to illness or death.
II. Hemolytic disease of the newborn (HDN)
Most cases of HDN that arise from an ABO incompatibility require no treatment. Cases of severe hemolysis that need exchange transfusions are less common. HDN caused by ABO antibodies occurs almost in infants of blood type A or B who are born to type O mothers. HDN tends to be relatively mild in nature because fetal RBCs don't express A and B antigen levels as adults. But the strength of fetal ABO blood group antigens can vary, and thus the degree of hemolysis and the severity of HDN may be unpredictable.
Notably, ABO antibodies are of major clinical significance for two reasons: (a) they are occurring naturally and found universally; (B) they are highly reactive. If you’re interested in these antigens, please contact us for more information.
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