35. Alloantigens of human erythrocytes. ABO blood group system. H blood group system. Biosynthesis of ABH antigens. Rhesus blood group system.
Explains how blood group alloantigens on erythrocytes trigger humoral responses, covering the AB0 system with its four groups, the H system and the Bombay phenotype, and the biosynthesis of A, B and H antigens by glycosyltransferases. It then moves to inheritance with a genotype table, naturally occurring antibodies with an antibody table, blood typing with three-well agglutination testing, compatible transfusion, and secretor versus non-secretor status with infection susceptibility. The Rhesus system is not covered.
Topic 35 of 64 in the MU Sofia syllabus for Biology (Dental Medicine, Year 1). The free AceMed note covers it in 3 sections, about 814 words (6 min of reading), with 13 flashcards.
In this note
Understanding Blood Group Alloantigens: AB0 and H-Blood Groups
Understanding Inheritance and Detection of AB0 Blood Groups
Understanding Compatible Transfusion and Secretor Status
Understanding Blood Group Alloantigens: AB0 and H-Blood Groups
Blood Groups Overview
Blood groups, or types, arise from specific carbohydrates and proteins on the surface of erythrocytes (red blood cells).
Unlike most cells, erythrocytes lack MHC antigens, which means they don’t typically trigger a cell-mediated immune response.
These surface markers, called alloantigens, can provoke a humoral immune response.
Blood group is determined by alleles of a single gene, making up various blood group systems, with AB0 and Rhesus being critical for transfusions.
AB0 Blood Group System
Discovery: Identified by Karl Landsteiner in the early 20th century when he noted that mixing blood from two people often led to agglutination unless their blood types matched.
Types:
A Group: Presence of A antigen only.
B Group: Presence of B antigen only.
AB Group: Presence of both A and B antigens.
O Group: Absence of both A and B antigens.
The AB0 system is based on the presence or absence of two antigens, A and B.
Chemical Nature of Blood Group Antigens
Blood group antigens are not proteins but oligosaccharides, parts of glycolipids and glycoproteins.
These components are part of the glycocalyx, which shields the cell membrane from external damage.
H-Blood Group System
The A, B, and H antigens are products of glycosyltransferase enzymes.
These antigens evolve from a common precursor oligosaccharide that concludes with a galactose residue.
In the maturation phase of erythrocytes, an enzyme called fucosyltransferase adds fucose to the galactose of this precursor, forming the H antigen.
The gene for this process is labeled as the H gene. A rare recessive allele, h, leads to the production of an inactive protein, causing those homozygous for h (hh) to lack the H antigen and exhibit only the precursor on their blood cells, leading to the Bombay phenotype.
• Biosynthesis of A and B antigens
o Gene AB0 has 3 main alleles: A, B and 0
o Alleles A and B encode enzymes transferase A and transferase B, resp.
o Allele 0 doesn't encode biologically active protein
o Both transferases use antigen H as substrate and add monosaccharide to subterminal galactose
▪ Transferase A adds N-acetylgalactosamine, producing antigen A
▪ Transferase B adds galactose, producing antigen B
o NB!
▪ Antigens A, B and H aren't encoded by respective genes! They're carbohydrates, genes don't encode carbohydrates, but proteins! The discussed genes encode enzymes (glycosyltransferases) that synthesize antigens
• Biosynthesis of ABH antigens
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