58. Blood group alloantigens. ABO and H blood groups.
Explains why blood group antigens trigger antibodies rather than T-cell responses, then covers the ABO system, its carbohydrate antigens and the FUT1, ABO and Se genes that build them. Includes a genotype, phenotype and antibody table, a population-frequency section, blood typing and transfusion rules, the rare Bombay phenotype, and secretor status with infection links. Four mnemonics summarise antigens, genes, Bombay and secretors.
Topic 59 of 105 in the MU Sofia syllabus for Biology (Medicine, Year 1). The free AceMed note covers it in 2 sections, about 1,290 words (9 min of reading), with 10 flashcards.
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Blood group alloantigens. ABO and H blood groups
MNEMONIC
Blood group alloantigens. ABO and H blood groups
Introduction to Blood Group Alloantigens
Blood transfusion is a type of transplantation, where the donor’s red blood cells are introduced into the recipient’s bloodstream.
Red blood cells (erythrocytes) do not express MHC (major histocompatibility complex) molecules, so they do not trigger a strong T-cell mediated immune response.
However, red blood cells carry polymorphic surface carbohydrates and proteins that differ among individuals and can stimulate a strong antibody (humoral) immune response.
These polymorphic molecules are called blood group antigens, and individuals with different forms of these antigens belong to different blood groups.
A blood group system refers to a set of antigens controlled by alleles of a single gene. There are several systems, but ABO and Rhesus are the most clinically significant.
ABO Blood Group System
Discovered by Karl Landsteiner, the ABO system was the first to be understood and is the most crucial for transfusion safety.
Landsteiner noticed that mixing blood from different people sometimes caused agglutination or clumping of red cells, revealing incompatibility.
There are four main blood types in this system: A, B, AB, and O.
Blood group A has antigen A.
Blood group B has antigen B.
Blood group AB has both antigens A and B.
Blood group O has neither antigen A nor B.
Chemical Nature and Biosynthesis of ABO Antigens
Unlike most antigens, ABO blood group antigens are carbohydrates, not proteins.
These antigens are part of glycoproteins and glycolipids on the outer surface of red blood cells, collectively forming the glycocalyx.
A common precursor oligosaccharide is present in all individuals.
This precursor is modified by glycosyltransferase enzymes, which are encoded by specific genes.
The FUT1 gene (also called the H gene) adds a fucose molecule to the precursor, forming the H antigen.
The ABO gene has three main alleles: A, B, and O.
The A allele encodes transferase A, which adds N-acetylgalactosamine to the H antigen, forming antigen A.
The B allele encodes transferase B, which adds galactose to the H antigen, forming antigen B.
The O allele is inactive and produces no enzyme, so the H antigen remains unmodified.
These enzymes determine which antigens appear on the red cell surface.
ABO Genotypes, Phenotypes, and Antibody Production
Individuals produce antibodies against the A or B antigens they do not have.
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