Life's Blood

CLASS NOTES

LEWIS BLOOD GROUP SYSTEM

There are distinct differences in regards to the Lewis Blood Group System

  • Manufactured by the tissues
  • Lewis antigens are secreted into body fluids
  • Absorbed onto red cells from the plasma

The Lewis Blood Group System is also similar to the ABO system

  • The antigens are part of the same oligiosaccharides that are part of the ABH antigens
  • The Lewis antigen (fucose) is added onto the N-acetyl-glucosamine that is just before the galactose where the fucose is added for the H antigen in the secretions.

Lewis Antigens

Alleles

The development of the Lewis antigens is controlled by two alleles of the Lewis blood group system.

  • Le  is  dominant and results in the presence of  Lewis antigen. 
  • The recessive le (absence of Lewis gene) is recessive and therefore 2 le/le needs to be inherited

Genotypes

Both Le/Le or Le/le result Lewis positive antigen.  Lewis antigen exists as either Lewis a (Lea) or Lewis b (Leb). Lewis negative results from le/le.

Phenotypes:

Lewis System Phenotypes and Their Incidence

(Modified from AABB Technical Manual, 2002, p. 287)

Reactions with Anti-

Phenotype Adult Phenotype Incidence in %
Lea Leb Whites Blacks
+ 0 Le(a+b-) 22 23
0 + Le(a-b+) 72 55
0 0 Le(a-b-) 6 22
+ + Le(a+b+) Rare Rare

Formation of Lewis Antigens

What Lewis antigens are  formed depends on interaction between Lewis (Le/le),  Secretor (Se) and H genes in the tissues to produce Lewis antigens in the secretions.

Lewis antigens have a similar structure to ABO antigens:

3. Formed at terminal sugars of Type I precursor substance made by tissue cell in plasma

a. If person has the Lewis gene, it adds fucose to second sugar from the end = Lea

b. If person is also a secretor, H gene adds fucose to terminal sugar of precursor substance = Leb antigen

c. If person NOT a secretor, no H added to precursor substance made by tissue cell, so it remains as Lea


4. NOT part of rbc membrane - synthesized by tissue cells, carried by plasma, adsorb onto rbc surface

5. Not present on newborn red cells

6. Can disappear during pregnancy

Lewis Antibodies

Types of antibodies
1. Anti-Lewisa
2. Anti-Lewisb
3. Anti-Lewisa and Lewisb

B. Almost always produced by Lewis a negative; Lewis b negative people

C. Naturally occurring but not regularly occurring (unexpected antibodies)

D. Frequently seen in pregnancy (due to loss of Lewis antigen during pregnancy)

E. IgM, therefore usually not clinically significant

1. Does not cross placenta

2. Reacts best at RT, but some MAY react at 37C

3. If anti-Lea present at 37C, may cause hemolytic transfusion reaction

4. Anti-Leb is usually clinically insignificant

III. Secretor Status

A. Secretor status controlled by Secretor (Se) gene

1. Secretor = Se/Se or Se/se (80%)

2. Non-secretor = se/se (20%)

B. Secretors have soluble A, B and H antigens in body fluids - plasma, tears, saliva

C. Can test saliva for presence of ABH antigens

D. Practical application of saliva testing:

1. Determine ABO type in patients with ABO discrepancies

2. Determine ABO type in patients massively transfused with another blood type

3. Can also use Lewis types to determine secretor status:

E. Lewis a positive, Lewis b negative = non-secretor

F. Lewis a negative, Lewis b positive = secretor

G. Lewis a negative, Lewis b negative = can't tell secretor status from Lewis types

H. Can only use this method if patient has not been heavily transfused recently - need to type patient red cells, not donor

IV. Testing for Secretor Status

A. Utilizes principle of AGGLUTINATION INHIBITION

1. Patient's saliva boiled and cleared

2. Cooled saliva mixed with reagent anti-A, anti-B and anti-H

3. If soluble A, B, or H antigens present in saliva, these will react with antibodies in reagent antiserum, and neutralize it, so no antibody available to agglutinate test cells

4. if no soluble A, B or H antigens in saliva, antibodies in reagent antiserum will not be neutralized, and will be free to react with test cells

B. Group A Secretor:

1. A antigens in saliva

2. Addition of reagent anti-A: antibodies tied up by soluble A antigens

3. Addition of known A cells results in no agglutination (no free A antibody to cause agglutination)




NO AGGLUTINATION = POSITIVE TEST FOR SECRETOR STATUS


C. Group A Non-Secretor:

1. NO ABO antigens in saliva





2. Addition of reagent Anti-A leaves free antibody in serum




3. Addition of known A cells causes agglutination.

AGGLUTINATION = NEGATIVE TEST FOR SECRETOR STATUS


OBJECTIVES - LEWIS BLOOD GROUP SYSTEM

1. State the alleles in the Lewis system

2. Explain how Lewis antigens are produced

3. Explain the difference between Lewis a and Lewis b

4. Explain the relationship between the secretor gene and Lewis antigen production

5. Describe the general characteristics of Lewis antigens, including presence at birth and effect of pregnancy

6. State the possible Lewis phenotypes, and their approximate percentages in the Caucasian population

7. State which Lewis phenotype is most likely to make antibodies in the Lewis system

8. Describe the typical characteristics of Lewis antibodies

9. State which of the Lewis antibodies is more clinically significant, and explain why.

10. Explain what is meant by the term "secretor"

11. Explain the principle of agglutination inhibition, and how it applies to saliva testing

12. Given the results of any saliva agglutination inhibition test, state whether or not the patient is a secretor, and determine his/her ABO type

13. Explain how secretor status can be determined from the Lewis phenotype

14. State limitations to the above system

15. Give two practical applications of saliva testing

Performance objectives:

1. Correctly perform and interpret the saliva test for agglutination inhibition, to determine your own secretor status

2. Correctly perform and interpret the antigen typing for Lewis a and Lewis b on your own red cells

3. Correlate the results of your Lewis typings with the results of you saliva testing

 

Clinical Microbiology Syllabus