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ABO

The ABO blood group system (ISBT 001) is arguably the most critical system in transfusion medicine due to the presence of potent, naturally occurring antibodies. Unlike many other systems that involve protein structures, ABO antigens are carbohydrates, specifically sugars added sequentially onto precursor chains found on the red blood cell surface. The specific sugar added is determined by inherited enzymes called glycosyltransferases, ultimately resulting in the familiar A, B, and H antigens that define a person’s ABO type

The Foundation: Precursor Substances

Everything starts with basic carbohydrate chains attached to proteins or lipids sticking out from the red blood cell membrane (glycoproteins or glycolipids). These precursor chains come in two main flavors relevant to ABO:

  • Type 1 Chains: Found primarily in plasma/secretions and on body cells. The terminal galactose (Gal) is linked to N-acetylglucosamine (GlcNAc) in a beta 1->3 linkage. (Galβ1->3GlcNAc-R)
  • Type 2 Chains: Found predominantly on the red blood cell surface. The terminal galactose (Gal) is linked to N-acetylglucosamine (GlcNAc) in a beta 1->4 linkage. (Galβ1->4GlcNAc-R)
    • Think of Type 2 chains as the primary building site on RBCs for ABO antigens

Step 1: Building the “H” Antigen - The Essential Intermediate

Before A or B antigens can be made, another antigen, the H antigen, must be formed on the precursor substance

  • Gene: FUT1 (also known as the H gene), located on chromosome 19
  • Enzyme: This gene codes for an enzyme called α-2-L-fucosyltransferase
  • Action: This enzyme transfers the sugar L-fucose from a donor molecule (GDP-fucose) onto the terminal galactose of the Type 2 precursor chain on the red blood cell surface
  • Result: The H antigen is formed. (Fucα1->2Galβ1->4GlcNAc-R)
  • Significance: The H antigen is the mandatory substrate (building block) for the A and B enzymes. No H antigen = No A or B antigen expression on RBCs (this leads to the Bombay phenotype, which we’ll discuss)
  • Alleles: The common functional allele is H (FUT1). The common non-functional allele is h, which produces an inactive enzyme. Most people are HH or Hh and make H antigen. The hh genotype results in the Bombay phenotype

Step 2: Adding the “A” and “B” Antigens - The Defining Step

The ABO gene, located on chromosome 9, determines which (if any) additional sugar is added to the H antigen

  • Gene: ABO gene
  • Alleles: Three main alleles: A, B, and O
  • Enzymes (Glycosyltransferases)
    • The A allele codes for α-3-N-acetylgalactosaminyltransferase. This enzyme transfers N-acetyl-D-galactosamine (GalNAc) onto the galactose of the H antigen. This creates the A antigen. (GalNAcα1->3[Fucα1->2]Galβ1->4GlcNAc-R)
    • The B allele codes for α-3-D-galactosyltransferase. This enzyme transfers D-galactose (Gal) onto the galactose of the H antigen. This creates the B antigen. (Galα1->3[Fucα1->2]Galβ1->4GlcNAc-R)
    • The O allele typically results from a mutation (often a frameshift deletion) that leads to a non-functional enzyme. No sugar is added to the H antigen. Therefore, Group O individuals have red cells rich in unmodified H antigen
  • Immunodominant Sugars: The final sugar added determines the antigen’s specificity:
    • A antigen: N-acetyl-D-galactosamine (GalNAc)
    • B antigen: D-galactose (Gal)
    • H antigen: L-fucose (present in A, B, and O phenotypes, but most abundant in O)

Genotype to Phenotype - Putting it Together

Genotype(s) FUT1 Enzyme ABO Enzyme Antigens on RBCs Phenotype
HH or Hh
AA or AO 		Functional A-transferase A and H (less H) A
HH or Hh
BB or BO 		Functional B-transferase B and H (less H) B
HH or Hh
AB 		Functional Both A- and B-transferases A, B, and H (least H) AB
HH or Hh
OO 		Functional Non-functional H only (most H) O
hh
Any ABO 		Non-functional A/B/None (irrelevant on RBCs) Precursor only (No H, A, B) Bombay (Oh)

ABO Subgroups: A1 and A2

The A phenotype is not uniform; the major subgroups are A1 and A2

  • A1: Represents about 80% of Group A (and AB) individuals
    • The A1 transferase is highly efficient and produces a large number of A antigen sites, some of which are branched structures (more complex)
  • A2: Represents about 20% of Group A (and AB) individuals
    • The A2 transferase results from slightly different A allele SNPs and is less efficient. It produces fewer A antigen sites per RBC, and these are primarily linear (less complex) structures
    • A2 cells have more unconverted H antigen than A1 cells
  • Clinical Relevance: Some A2 (and especially A2B) individuals (~1-8% of A2, ~22-35% of A2B) may produce an anti-A1 antibody. This antibody reacts with A1 cells but not their own A2 cells. It’s usually a cold-reacting IgM and only clinically significant if reactive at 37°C (rare). The lectin from Dolichos biflorus specifically agglutinates A1 cells, helping differentiate A1 from A2

Location of ABO Antigens

ABO antigens aren’t just on red blood cells!

  • Red Blood Cells: Synthesized on Type 2 precursor chains
  • Other Cells: Widely expressed on the surface of endothelial cells (lining blood vessels) and many epithelial cells throughout the body. Also present on platelets (adsorbed from plasma) and lymphocytes
  • Secretions (Soluble Antigens): ABO antigens can also be found as soluble glycoproteins in body fluids like plasma, saliva, tears, sweat, etc. This depends on inheriting the Secretor gene (FUT2)
    • Secretor Gene (FUT2): Also on chromosome 19, codes for another α-2-L-fucosyltransferase that acts primarily on Type 1 chains in secretory tissues
    • Individuals who are SeSe or Sese (about 80% of the population) express soluble H antigen in secretions. If they also have A or B genes, they will secrete soluble A or B antigens, respectively
    • Individuals who are sese do not secrete H, A, or B antigens, regardless of their ABO type
    • Note: The FUT1 gene controls H on RBCs; the FUT2 gene controls H (and thus A/B) in secretions

The Bombay Phenotype (Oh)

This rare phenotype is critical to understand:

  • Genotype: Homozygous recessive at the H locus (hh) and any ABO genotype
  • Biochemistry: Lack functional FUT1 enzyme. Cannot produce H antigen on their red cells
  • Consequence: Since H is the substrate for A and B enzymes, individuals cannot make A or B antigens on their RBCs, even if they have the A or B genes
  • Serology: Types as Group O in routine forward typing (no reaction with anti-A or anti-B)
  • Antibodies: Crucially, Bombay individuals produce a potent, wide-thermal range anti-H antibody (in addition to anti-A and anti-B, unless they have A or B genes which might prevent this). This anti-H reacts strongly with all red cells except other Bombay cells (because all non-Bombay cells have H antigen, with Group O having the most)
  • Transfusion: Can only receive blood from other Bombay donors

ABO Antibodies

A unique feature of the ABO system is the predictable presence of antibodies in the plasma/serum directed against the antigen(s) absent from the individual’s own red cells

  • “Naturally Occurring”: Usually appear in the first 3-6 months of life, likely stimulated by environmental exposure to A-like and B-like antigens on bacteria, pollen, etc. Not typically present at birth
  • Reciprocal Relationship (Landsteiner’s Rule)
    • Group A individuals have Anti-B
    • Group B individuals have Anti-A
    • Group O individuals have Anti-A, Anti-B, and Anti-A,B
    • Group AB individuals have no ABO antibodies
    • Bombay individuals have Anti-A, Anti-B, Anti-A,B, and Anti-H
  • Immunoglobulin Class: Primarily IgM, which are large molecules, excellent at agglutination (visible clumping) and potent activators of the classical complement pathway. This leads to rapid intravascular hemolysis if incompatible blood is transfused
  • IgG Component: Group O individuals often have a significant IgG component of anti-A and anti-B, in addition to IgM. This IgG anti-A,B can cross the placenta and is the most common cause of ABO Hemolytic Disease of the Fetus and Newborn (HDFN). Group A and B individuals typically have mostly IgM antibodies
  • Clinical Significance: ABO antibodies are the most clinically significant blood group antibodies, capable of causing acute, severe, potentially fatal hemolytic transfusion reactions (HTRs)

Key Terms

  • Glycosyltransferase: An enzyme that transfers a sugar molecule from a donor to an acceptor molecule (e.g., A-transferase, B-transferase, H-transferase)
  • Immunodominant Sugar: The terminal sugar molecule on an antigen that defines its specificity and is primarily recognized by an antibody (GalNAc for A, Gal for B, Fucose for H)
  • Precursor Substance: The basic carbohydrate chain (Type 1 or Type 2) upon which H, A, and B antigens are built
  • H Antigen: The antigen formed by adding fucose to a precursor chain; it is the substrate for A and B antigen synthesis on red cells. Controlled by FUT1 (H gene)
  • A Antigen: Antigen formed by adding N-acetylgalactosamine (GalNAc) to the H antigen
  • B Antigen: Antigen formed by adding D-galactose (Gal) to the H antigen
  • Amorph: A gene that does not produce a detectable product (e.g., the O allele produces a non-functional enzyme)
  • Bombay Phenotype (Oh): Rare phenotype resulting from the hh genotype; individuals lack H, A, and B antigens on their red cells and produce anti-H
  • Secretor (Se) Gene (FUT2): Gene controlling the expression of H antigen (and thus A/B antigens) in body secretions via action on Type 1 chains
  • Lectin: A protein, usually of plant origin, that binds specifically to carbohydrates (e.g., Dolichos biflorus lectin binds A1 antigen)
  • Naturally Occurring Antibody: Antibody present in the serum/plasma without known prior exposure to the corresponding red cell antigen (typically IgM, like anti-A and anti-B)
  • Landsteiner’s Rule: Healthy individuals possess ABO antibodies in their serum/plasma directed against the ABO antigen(s) absent from their own red cells