Lewis

The Lewis blood group system (ISBT 007) is fascinating because, unlike ABO or Rh, Lewis antigens aren’t manufactured by the red blood cell itself. Instead, they are soluble carbohydrate antigens found in plasma and secretions that passively adsorb onto the red cell membrane. This system is intricately linked with the Secretor system (FUT2 gene) and ABO genetics, with the interplay between the Lewis (FUT3) and Secretor (FUT2) fucosyltransferase enzymes determining whether Le^a or Le^b antigens are formed on Type 1 precursor chains

**Genetics: Two Key Players (FUT3 and FUT2)**

The expression of Lewis antigens depends on the interplay of two genes, both located on chromosome 19:

Lewis Gene (FUT3 or Le)
- Codes for an enzyme called α-1,4-L-fucosyltransferase (sometimes written as Fucosyltransferase 3 or FucT-III)
- Function: Adds a fucose sugar to the N-acetylglucosamine (GlcNAc) residue of Type 1 precursor chains
- Alleles
  - Le: Functional allele, produces the enzyme
  - le: Amorph (non-functional) allele, common mutation results in no active enzyme. le is recessive to Le
Secretor Gene (FUT2 or Se): (We discussed this with ABO secretions)
- Codes for an enzyme called α-1,2-L-fucosyltransferase (Fucosyltransferase 2 or FucT-II)
- Function: Adds a fucose sugar to the terminal galactose (Gal) residue of Type 1 precursor chains (primarily in secretory tissues) and also Type 2 chains (relevant for H on RBCs via FUT1)
- Alleles
  - Se: Functional allele, produces the enzyme active in secretory tissues
  - se: Amorph allele, results in no active enzyme in secretory tissues. se is recessive to Se

Biochemistry: Building the Lewis Antigens (on Type 1 Chains)

Lewis antigens are built on Type 1 precursor chains (Galβ1->3GlcNAc-R), which are primarily found in secretions and plasma as glycolipids or glycoproteins

Step 1: Formation of Le^a Antigen
- If an individual has at least one functional Le gene (FUT3), the α-1,4-L-fucosyltransferase adds a fucose to the GlcNAc of the Type 1 chain
- Result: Le^a antigen is formed
- Structure: Galβ1->3[Fucα1->4]GlcNAc-R
- Requires only the Le enzyme
Step 2: Formation of H Antigen (Type 1)
- If an individual has at least one functional Se gene (FUT2), the α-1,2-L-fucosyltransferase adds a fucose to the terminal Galactose of the Type 1 chain (in secretory tissue)
- Result: H antigen (Type 1) is formed
- Structure: Fucα1->2Galβ1->3GlcNAc-R
- Requires only the Se enzyme
Step 3: Formation of Le^b Antigen - The Interaction!
- If an individual has both a functional Le gene AND a functional Se gene, both enzymes are present
- The Se gene enzyme adds fucose to the Galactose (creating H Type 1)
- The Le gene enzyme adds fucose to the GlcNAc of the same chain
- Result: Le^b antigen is formed, containing two fucose molecules on adjacent sugars
- Structure: Fucα1->2Galβ1->3[Fucα1->4]GlcNAc-R
- Requires BOTH the Le and Se enzymes acting on the same Type 1 chain
- Competition: The Se gene enzyme (FucT-II) acts preferentially on the Type 1 chain. Once H is formed, the Le enzyme (FucT-III) readily adds the second fucose to create Le^b. This efficient process means that in individuals with both Le and Se genes, most precursor chains are converted to Le^b, leaving very little Le^a available

Lewis Antigens on Red Blood Cells: Adsorption

Lewis antigens are synthesized primarily by epithelial cells of mucous tissues and are present as glycolipids in plasma
These plasma Lewis glycolipids passively adsorb onto the red blood cell membrane, intercalating into the lipid bilayer
The amount and type of Lewis antigen on the RBCs directly reflect what is present in the individual’s plasma
Because they are adsorbed, Lewis antigen expression can decrease during pregnancy or certain diseases, and donor Lewis antigens can elute off transfused red cells over time

Common Lewis Phenotypes

The RBC phenotype depends on the interaction of the Le and Se genes:

Le(a+b-)
- Genotype: Le/ (LeLe or Lele), sese
- Enzymes: Has functional Le enzyme, lacks functional Se enzyme
- Antigens Formed: Can only make Le^a antigen (adds fucose to GlcNAc). Cannot make H Type 1 or Le^b in secretions/plasma
- Result: Le^a is present in plasma/saliva and adsorbs onto RBCs
Le(a-b+)
- Genotype: Le/ (LeLe or Lele), Se/ (SeSe or Sese)
- Enzymes: Has both functional Le and Se enzymes
- Antigens Formed: Both enzymes act on Type 1 chains. Le^b is formed efficiently. Very little free Le^a remains. H Type 1 is also formed
- Result: Le^b is the predominant antigen in plasma/saliva and adsorbs onto RBCs. The RBCs type as Le(a-b+). (This is the most common phenotype in many populations)
Le(a-b-)
- Genotype Possibility 1: lele, any Se (SeSe, Sese, or sese)
  - Enzymes: Lacks functional Le enzyme. Cannot add fucose to GlcNAc
  - Antigens Formed: Cannot make Le^a or Le^b. May make H Type 1 if Se is present, but no Lewis antigens
  - Result: No Lewis antigens in plasma/saliva to adsorb onto RBCs
- Genotype Possibility 2: Le/, sese OR Le/, Se/ (Transiently)
  - Explanation: Newborns may initially type Le(a-b-) as Lewis glycolipid synthesis and adsorption take time. Pregnant women often show a temporary decrease or loss of Lewis antigens on their RBCs, typing as Le(a-b-)
Le(a+b+)
- Occurrence: Very rare in adults, more common in infants/young children
- Explanation: May represent a transitional stage where both Le^a and Le^b glycolipids are present in plasma and adsorb. Possibly due to less efficient conversion to Le^b initially or variations in enzyme activity. In adults, it’s extremely uncommon because the conversion to Le^b is usually very efficient in Le/Se individuals

Lewis Antibodies (Anti-Le^a, Anti-Le^b)

Formation: Often “naturally occurring,” meaning they appear without known red cell stimulus (transfusion or pregnancy). Usually found in individuals with the Le(a-b-) phenotype. Le(a+b-) individuals do not make anti-Le^b
Immunoglobulin Class: Primarily IgM
Reactivity: Typically react best at room temperature or below in saline (immediate spin). May sometimes be detected at 37°C and/or the AHG phase, especially with enzyme-treated cells
Clinical Significance
- Generally considered clinically insignificant if they react only below 37°C. They usually do not cause hemolytic transfusion reactions (HTRs) or Hemolytic Disease of the Fetus and Newborn (HDFN)
- Why usually not significant?
  1. Often IgM, which doesn’t cross the placenta (No HDFN)
  2. Antigens are poorly developed on fetal/newborn RBCs (No HDFN)
  3. Antibodies are often neutralized by soluble Lewis antigens present in the donor plasma transfused with the RBCs
  4. Lewis antigens can elute off transfused Le+ cells in an Le- recipient
  5. Optimal reactivity is usually below body temperature
- Exception: RARELY, a Lewis antibody (especially anti-Le^a) may show reactivity at 37°C and cause complement activation, potentially leading to hemolysis in vitro or, very infrequently, in vivo. Antibodies reacting at 37°C/AHG phase require more careful consideration and provision of antigen-negative units
Neutralization: Lewis antibodies can often be neutralized by incubating the patient’s plasma with commercial Lewis substance or sometimes even pooled plasma/saliva (which contain soluble Lewis antigens), preventing reactivity with test cells. This can be a useful identification technique

Summary Table: Lewis System Genetics and Phenotypes

Genotype	Functional Enzymes	Antigens in Plasma/Secretions	RBC Phenotype	Typical Antibodies Made
Le/, sese	Le only	Le^a	Le(a+b-)	None
Le/, Se/	Le and Se	Le^b, H, (trace Le^a)	Le(a-b+)	None
lele, any Se	None (or Se only)	None (or H only)	Le(a-b-)	Anti-Le^a, Anti-Le^b, Anti-Le^ab
Le/, Se/ (Transient/Infant)	Le and Se	Le^a, Le^b, H	Le(a+b+) or Le(a-b-)	Variable

Key Terms

Adsorption: The process by which soluble substances (like Lewis glycolipids) in the plasma attach to the surface of red blood cells
Type 1 Precursor Chain: The carbohydrate chain (Galβ1->3GlcNAc-R) found primarily in secretions and plasma, serving as the substrate for Lewis and Secretor enzymes
FUT3 (Lewis Gene): Gene encoding α-1,4-L-fucosyltransferase, responsible for adding fucose to GlcNAc to create Le^a antigen
FUT2 (Secretor Gene): Gene encoding α-1,2-L-fucosyltransferase, responsible for adding fucose to Galactose to create H Type 1 antigen (in secretions) and influencing Le^b formation
Le^a Antigen: Lewis antigen formed by the action of the Le gene enzyme only
Le^b Antigen: Lewis antigen formed by the sequential or combined action of both the Le and Se gene enzymes on the same Type 1 chain
Glycolipid: A lipid molecule with attached carbohydrate chains (how Lewis antigens exist in plasma)
Neutralization: A technique used in antibody identification where soluble antigen is added to plasma to inhibit antibody reactivity with test cells

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