ABO and H Blood Group Systems and Secretor Status

Landsteiner’s Rule (law)

Healthy individuals possess ABO antibodies to the ABO blood group antigens ABSENT from their RBCs

ABO Antigen

• can be intrinsic to the RBC membrane or soluble (body fluids)

• detected in the embryo as early as 5 to 6 weeks’ gestation

• Newborns’ RBCs have fewer numbers and partially developed antigens

• Full expression occurs at about 2 to 4 years of age

ABO and H Antigen Genetics

• Genes at 3 separate loci influence presence of ABO antigens: ABO, H, Se

  • presence or absence of ABH antigens on RBC membrane = H gene

  • presence or absence of ABH antigens in secretions = Se gene

Genetics: H, Se, ABO genes

• H Gene: H and h alleles (h is an amorph)

  • Bombay phenotype lacks H antigen (hh)

• Se gene: Se & se alleles (se is an amorph)

• ABO genes: A, B and O alleles

What is the basic precursor for RBC antigens, A, B and H?

Oligosaccharide Chain

• Attached to a protein or lipid carrier molecule

H Antigen

• Foundation for A and/or B antigens

• codes for glucosyltransferase

  • transfers the immunodominant sugar, L-fucose

A and B Antigens

• “A” gene = transferase adds the immunodominant sugar, N-acetylgalactosamine, to H antigen (terminal sugar)

  • Gene Product: N-acetylgalactosaminyltransferase

• “B” gene = transferase adds the immunodominant sugar, D-galactose, to H antigen (terminal sugar)

  • Gene Product: D-galactosyltransferase

Antigen Concentration

Certain blood types possess more H antigen than others

• Group O individuals have many H antigen sites because they have no A or B antigens

  • O > A2 > B > A2B > A2 > A1B

Subgroups of A Type: A1 and A2

• Both react strongly w/ reagent anti-A (3+ to 4+)

• Lectin Dolichos biflorous distinguishes A1 from A2 red cells

  • Agglutinates w/ A1, but NOT A2

• 80% of A or AB individuals are A1

• 20% are A2 & A2B

Rare subgroup of the A type may be present if:

• Weak or no agglutination with commercial anti-A and anti-A,B occurs

• Anti-A1 is present

• Anti-H causes strong agglutination

What is more common, A or B subgroups?

A subgroups are more common than B subgroups; B subgroups typically react weakly with anti-B reagents.

Why is it important to identify subgroups?

Preventing transfusion rxns

• If a weak subgroup is missed in a donor, the incorrect blood group could be given to the recipient

a group A subgroup in the donor is classified as group O. the group O recipient receives the unit, what will occur?

a transfusion reaction; the recipient’s anti-A antibody reacts with the donor cells

ABO Antibodies

Non-RBC stimulated, or naturally occurring

• Ab production is unrelated to an RBC Ag

• Results from exposure to A and B like Ags in the environment (e.g., normal bacterial flora)

When are ABO Abs detected?

• may not be detected until 3 to 6 months of age

• Titers reach max. levels by 5 to 10 yrs of age and DECREASE as an individual grows older

Clinical Significance of ABO Abs

most anti-A & anti-B in group A or B individuals are predominantly IgM → clinically significant bc they are able to bind complement

• React optimally during immediate-spin (IS) crossmatching

• Anti A,B antibody found in group O individuals

  • Cross reacts w/ both A and B Ags

ABO Antibody Class

Anti-A1 Antibody

• Produced by subgroups of the A type

• Has SPECIFICITY to the A1 antigen but will not agglutinate A2

• Not clinically significant

• May cause incompatible crossmatches on IS testing

Routine ABO Phenotyping

Two components of ABO phenotyping, required testing on donor and recipient blood

• Forward grouping

  • RBCs are tested for ABO antigens

  • Determines ABO phenotype

• Reverse grouping

  • Serum or plasma is tested for ABO antibodies

  • Acts as a control for RBC testing

ABO Phenotype Reactions

Transfusion Selection: RBC vs Plasma

RBC:

• Group O: universal donor

• Group AB: universal recipient

Plasma:

• Group AB: universal donor

• Group O: universal recipient

RBC transfusion

• ABO-identical unit is preferred, followed by ABO-compatible unit for RBC transfusions

• Incompatible RBCs can cause an acute

  hemolytic transfusion reaction

• ABO-identical unit is required in whole blood transfusions

Plasma transfusion

• ABO-identical unit is preferred or compatible (reverse of RBC transfusions)

• Donor unit antibodies must be compatible w/ recipient’s RBCs

ABO Compatibility

ABO Discrepancies

occur when the forward grouping does not agree with the reverse grouping

• A discrepancy may be present if:

  • Agglutination is weaker than expected

  • Expected reactions are missing

  • Extra reactions are noted

Practical Application: Investigating ABO Technical Errors

Sample-Related Discrepancies: Red Cell Testing Problems

Sample-Related Discrepancies: Serum/Plasma Testing Problems

Extra Antigens

• Group A with acquired B

  • Deacytlating Enzyme

• B(A) phenotype:

  • similar to acquired B (pt is group B)

• Polyagglutination

  • Hidden Ags

• Non-specific aggregation causes

  • rouleaux and Wharton’s jelly

Group A with acquired B and B(A) phenotype: Extra Antigens

• group A immunodominant sugar is altered by a bacterial deacetylating enzyme

• Resembles group B and cross-reacts with anti-B

• B(A) phenotype: group B patient

Polyagglutination and Nonspecific aggregation cause: Extra Antigens

• Polyagglutination: A hidden antigen on the RBCs is exposed and reacts with most human sera

• Nonspecific aggregation causes

  • Rouleaux: abnormal amounts of serum protein

  • Wharton’s jelly: gelatinous tissue contaminant in cord blood

Missing or Weak Antigens

ABO subgroups: weak/no reactivity with anti-A/anti-B reagents.

• Leukemia/Hodgkin’s disease: weakened A and B antigen expression.

• Resolution: Check patient history, repeat with anti-A,B to enhance subgroup

Mixed-Field Reactions Causes

contain both agglutinated and unagglutinated cells and is caused by

• Two distinct cell populations (group O RBCs transfused to a group A, B, or AB individual)

• Bone marrow transplant

• Stem cell transplant

• A3 phenotype

  – Tn-polyagglutinable RBC

Extra Antibodies Causes

occur as extra agglutination reactions in the reverse testing because

• Anti-A1

• Cold alloantibodies

  • Abs specific for autologous Ags that react at room temp. or below

• Cold autoantibodies

  • Abs specific for human RBC Ags that react at room temp. or below

• Rouleaux: false-positive agglutination

Resolving Rouleaux

When are Missing or Weak Antibodies shown?

Show weak or negative agglutination in the reverse phase of testing

• rxn may be explained by investigating the patient history, age, diagnosis, and immunoglobulin levels

  • Newborns and elderly people have reduced Abs

  • Pathologic states that lower immunoglobulin levels

Examples of Discrepancies

Bombay Phenotype

• RBCs lack the H antigen (hh).

• Classified as group O (no H, A, or B antigens present).

• Serum contains anti-H, anti-A, and anti-B.

• Transfusion:

  • Only autologous units or rare donor files can be used

  • Group O RBCs cannot be given because the H antigen present

Secretor Status

• Two allele genes at this locus: Se and se

• Se is responsible for H substance in body secretions (e.g., saliva)

  • H is converted to A or B by glycosyltransferases

• 80% of the population are secretors

  • SeSe (homozygous)

  • Sese (heterozygous)

• 20% are nonsecretors (sese)

Secretor Status