Lewis Antigens and Genes

Lewis Antigens

• These are not an integral part of the red blood cell (RBC) membrane; instead, they are found in secretions
• Shares same precursor as ABO antigens, which is Type 1 paragloboside
  • This precursor is present in saliva, sweat, tears, and breast milk
• Expressed on RBC phenotype only if adsorbed
  • For example, Le^a and Le^b are produced in secretions
  • If the red cell adsorbs Le^a, the resulting phenotype is Le(a+b-)
  • if Le^b is adsorbed, the resulting phenotype is Le(a-b+)
• Can easily disintegrate/dissociate
  • Not permanent due to neutralization in plasma or secretions

Genes and Enzymes

• Genes involved are located on Chromosome 19
• Le gene encodes Alpha 1,4-L-fucosyltransferase
  • This enzyme transfers fucose sugar to paragloboside
• Se gene encodes Alpha 1,2-L-fucosyltransferase
• Le and Se genes compete in adding fucose to paragloboside:
  • If Le adds fucose first, Lea is formed and Se cannot add fucose, resulting in no LeB
  • If Se adds fucose first, Le can still add fucose, forming Leb
  • The H antigen can be converted to LeB if the Le gene adds fucose (alpha 1-4)
• ABO and Lewis antigens can coexist because they share the same Type 1 paragloboside precursor

Alleles and Phenotypes

• If the Le gene is the only one present, it forms the Le^a phenotype/antigen
• If both Le and Se genes are present, the Le^a gene will not be produced; instead, the Le^b gene is formed
• If the H gene is the only one present, it forms the H phenotype/antigen
• If both Le and H genes are present, the Le^a antigen will still be formed
  • The Le gene and Se gene should be present in order to create Le^b

H/Se Gene

• H antigen/Se gene, both sharing the same precursor + 1 Fucose = expression of abundant H antigen

A Gene

• H antigen/Se gene + 1 fucose + N-acetyl-D-galactosamine = A antigen expression
  • A and B antigens do not express in the absence of H antigen
• A antigen + Le gene (big) + 1 Fucose = ALeb (ALey) expression
  • A antigen originally has 1 fucose; the addition of another fucose (2 fucose present) leads to ALeb (ALey) expression
  • Phenotype is A, Le(a-b+)
• A antigen + le gene (small) with no addition of extra fucose = A antigen expression
  • A antigen expression is the same since it originally only requires 1 fucose

O Gene

• No addition of sugar is present

h/se Gene

• h antigen/se gene (small) gives no addition of fucose
• h antigen/se gene (small) + Le gene (Big) + 1 Fucose = Lea(Lex)
• h/se (null) phenotypes do not code for fucose therefore only 1 fucose is present from the Le gene
  • Phenotype is Le(a+b-)
• h antigen/se gene (small) + le gene (small) gives no addition of fucose

Lewis Antigen: Soluble Antigens

• Caused by dissociation/neutralization in secretions or plasma
• Occurs in 2 forms:
  • Glycoproteins
    • Cannot adsorb onto RBC membranes
  • Glycolipids
    • Can adsorb onto RBC membranes
• Resistant to ficin, papain, Dithiothreitol (DTT), and Glycine-acid EDTA:
  • Due to the antigens being absent in the RBCs and are only found in secretions
• Inheritance depends on the Se and Le genes and is dominant

se and le

• se and le alleles are amorph, resulting in a null phenotype
• Consists of two antigens, Lea, produced if the Le gene is present, and Leb, produced if both Le and Se genes are present
  • Leb is more common because there is more Se than Le, giving Se a better chance of adding fucose first

Le act

• Le act in competition with Se gene
  • The Le gene has more steric hindrance than the Se gene
  • If the Le gene adds fucose first, the Se gene cannot add fucose to the paragloboside and Le^a cannot be converted into Le^b

Alpha-4-L-fucosyltransferase

• Transfers L-fucose to the GlcNac of the type 1 chain and forms Lea

Se and ABH genes

• Intimately associated with Se and ABH genes in the formation of Leb
  • Share the same precursor (type 1 paragloboside)

Lewis Antigen

• If the Le gene is inherited, Lea substance is produced
• Le and Se genes must be inherited to convert H to Leb

Lew

• "w" stands for weak
  • Characterized by a non-functional or partially functional transferase
  • Defect with its transferase enzyme
  • Le antigens are present in the secretion but absent in the plasma and RBCs

Development of the Le Antigen

• Le antigen is not natural occurring
• Detectable in the RBCs approximately 10 days after birth
  • Lewis glycolipids are not detectable in plasma until 10 days after birth
• Lea & Leb glycoproteins are detectable in saliva at birth

Lewis antigen Development Table

• Phenotype changes occur because the Le antigen is present in the secretions, the phenotype will depend on what the RBC will absorb

Le (a-b-)

• Incubation of Le (a-b-) with the plasma of Le (a+b-) or Le (a-b+) can transform it:
  • Incubate Le (a-b-) to the PLASMA of Le (a+b-) or Le (a-b+) plasma
  • If the plasma of patient 1 has been incubated to the plasma of patient 2, after the incubation the red cell of patient 1 will be Le (a+b-)
    • in this case, patient 1 copied the phenotype of patient 2 because it was adsorbed by the RBC of patient 1
  • This transformation results from exposure to the plasma of someone with Le (a+b-) or Le (a-b+) phenotype, allowing adsorption and phenotypic conversion.
    • Le (a-b-) → Le (a-b+)

Notes

• With saliva as a source of Lewis substances, Le (a-b-) RBCs cannot be converted to Lewis (+) phenotypes b/c Lewis substances in saliva, being glycoproteins, are not adsorbed onto the RBC membranes

Lewis Antibodies

• Naturally occurring IgM antibodies:
  • IgM antibodies cannot cross the placenta, produced by Le(a-b-) individuals (usually null phenotype)
  • May activate complement and cause in-vivo and in-vitro hemolysis
  • Occur frequently in pregnant women due to diluted Le a and Le b because of plasma volume expansion
  • React more in group O than A or B because group O has more H antigen
  • Can be neutralized by Le antigen in plasma they are usually not detected
  • Can dissociate

Antibodies Produced

• Anti-Lea: Most commonly encountered antibody
  • Leb Antigen produces Anti-Lea (antibody)
  • Often IgM; sometimes IgG
• Anti-Leb: Not common and not as reactive
  • Anti-LebH is dependent to H antigen
  • Anti-Lebl is NOT dependent to H antigen
• Anti-Leab: Formerly known as anti-Lex
  • Is named as “ANTI-Leab” “ because it agglutinates in both phenotypes:
    • Le(a+b-)
    • Le(a-b+) cells
  • This antibody will only be formed, when the patient’s blood type is null or Le(a-b-)

Clinical significance

• CANNOT CAUSE HDFN
  • Le antibodies are IgM (cannot cross through placenta)
  • Le antigens are not fully developed at birth
    • It will only be develop 10 days after birth
• CONSIDERED INSIGNIFICANT IN BLOOD TRANSFUSION
  • Le antibody can be easily neutralized by Le antigen in plasma
  • IMPORTANT: detects @ 37°C and Coombs phase (+)

Other Lewis Antigens

Important note

• Le antigen can readily dissociate from RBCs