4.2 - ABO Blood Group System

Which of the following distinguishes the A1 blood group from the A2 blood group?

A. A2 antigen will not react with Anti-A, A1 will react strongly (4+)

B. An A2 person may form Anti-A1; an A1 person will not form Anti-A1

C. An A1 person may form Anti-A2, an A2 person will not form Anti-A1

D. A2 antigen will not react with Anti-A from a nonimmunized donor; A1 will react with any Anti-A

B. An A2 person may form Anti-A1; an A1 person will not form Anti-A1

• A1 and A2 are subgroups of group A differing in antigen structure and quantity.

• A1 RBCs have more A antigen sites and react with Anti-A1 lectin (Dolichos biflorus).

• A2 RBCs have fewer antigen sites and may occasionally produce Anti-A1.

Summary
A2 individuals can form Anti-A1, while A1 individuals do not — key distinction between A1 and A2 subgroups.

A patient's serum is incompatible with O cells. The patient RBCs give a negative reaction to Anti-H lectin. What is the most likely cause of these results?

A. The patient may be a subgroup of A

B. The patient may have an immunodeficiency

C. The patient may be a Bombay phenotype individual

D. The patient may have developed alloantibodies

C. The patient may be a Bombay phenotype individual

• Serum incompatible with O cells → presence of anti-H (O cells express abundant H).

• Patient RBCs nonreactive with Anti-H lectin → no H antigen on RBCs → hh (Bombay).

• Subgroup A would still have H; alloantibodies don’t explain Anti-H lectin neg RBCs.

Summary
Incompatibility with O cells + Anti-H lectin negative RBCs → Bombay (hh) with anti-H.

What antibodies are formed by a Bombay phenotype individual?

A. Anti-A and Anti-B

B. Anti-H

C. Anti-A, B

D. Anti-A, B, and H

D. Anti-A, B, and H

• Bombay phenotype (hh) lacks H antigen, so A and B antigens cannot be formed.

• Consequently, serum contains anti-A, anti-B, and anti-H — all typically strong, complement-binding IgM.

• These antibodies react with all routine donor RBCs, including group O.

Summary
Bombay individuals produce anti-A, anti-B, and anti-H due to complete absence of H antigen on RBCs.

Acquired B antigens have been found in:

A. Bombay phenotype individuals

B. Group O persons

C. Persons of all blood groups

D. Group A persons

D. Group A persons

• Acquired B phenotype occurs when bacterial enzymes (often from gram-negative infections) modify A antigen’s terminal sugar, making it react with anti-B.

• Seen only in group A (or AB) individuals, as A substance is required for alteration.

• Reaction disappears once infection resolves or red cells are replaced.

Summary
Acquired B appears in group A individuals when bacterial deacetylation of A antigen mimics B reactivity.

Blood is crossmatched on an A-pos person with a negative antibody screen. The patient received a transfusion of A-pos RBCs 3 years ago. The donors chosen for cross matching were A-positive. Cross matching was run on the automated analyzer and yielded 3+ incompatibility. How can these results be explained?

A. The patient has an antibody to a low-frequency antigen

B. The patient has an antibody to a high-frequency antigen

C. The patient is an A2 with Anti-A1

D. The patient is an A1 with Anti-A2

C. The patient is an A2 with Anti-A1

• Antibody screen neg because screening cells are group O (no A1 antigen) → anti-A1 not detected.

• Crossmatch 3+ with many A-positive donors because most are A1 → anti-A1 reacts.

• Fits A2 patient producing anti-A1 (often cold, IS/RT) causing automated crossmatch incompatibility.

• Confirm/resolve: test with Dolichos biflorus (A1 lectin), provide A2 or O compatible units.

Summary
Screen missed anti-A1 (O cells); crossmatch flagged A1 donor cells → A2 with anti-A1.

A patient's RBCs forward as group O, serum agglutinates B cells (4+) only. Your next step would be:

A. Extend reverse typing for 15 minutes

B. Perform an antibody screen, including room-temperature incubation

C. Incubate washed RBCs with Anti-A1 and Anti-A,B for 30 minutes at room temperature

D. Test patient's RBCs with Dolichos biflorus

C. Incubate washed RBCs with Anti-A1 and Anti-A,B for 30 minutes at room temperature

• Forward = O; reverse = anti-B only → pattern suggests A subgroup (weak/undetected A).

• Anti-A,B detects weak A better than anti-A; Anti-A1 lectin clarifies A1 vs non-A1.

• This resolves a forward/reverse discrepancy consistent with a weak A phenotype.

Summary
Reverse suggests group A; forward missed weak A. Room-temp incubation with anti-A,B ± anti-A1 is the next step.

Which typing results are most likely to occur when a patient has an acquired B antigen?

A. Anti-A 4+, Anti-B 3+, A1 cells neg, B cells neg

B. Anti-A 3+, Anti-B neg, A1 cells neg, B cells neg

C. Anti-A 4+, Anti-B 1+, A1 cells neg, B cells 4+

D. Anti-A 4+, Anti-B 4+, A1 cells 2+, B cells neg

C. Anti-A 4+, Anti-B 1+, A1 cells neg, B cells 4+

• Acquired B (typically in group A patients with GI infection) causes weak Anti-B reactivity on forward typing due to A antigen deacetylation mimicking B.

• Reverse typing remains typical of group A: A1 cells neg (no anti-A), B cells 4+ (anti-B present in serum).

Summary
Group A with weak Anti-B on forward and strong anti-B in reverse → acquired B pattern.

Which blood group has the least amount of H antigen?

A. A1B

B. A2

C. B

D. A1

A. A1B

• H antigen is the precursor for A and B antigens; as A and B sugars are added, H sites are converted.

• A1B cells express both A and B antigens in high density, leaving very little unconverted H.

• O > A2 > B > A1 > A1B in decreasing H amount.

Summary
A1B group has the least H antigen because nearly all H sites are converted to A and B antigens.

What type RBCs can be transfused to an A2 person with Anti-A1?

A. A only

B. A or O

C. B

D. AB

B. A or O

• A2 with Anti-A1 → cannot receive A1 RBCs, as antibody may react.

• A2 RBCs (same subgroup) are compatible, and group O RBCs (no A1 antigen) are universally compatible for the A system.

• B and AB RBCs** are incompatible** due to B antigen.

Summary
A2 or O RBCs are safe for A2 individuals with Anti-A1, avoiding exposure to A1 antigen.

What should be done if all forward and reverse ABO results as well as the autocontrol are positive?

A. Wash the cells with warm saline, and autoadsorb the serum at 4C

B. Retype the sample using a different lot number of reagents

C. Use polyclonal typing reagents

D. Report the sample as group AB

A. Wash the cells with warm saline, and autoadsorb the serum at 4 °C

• All forward, reverse, and autocontrol positive → suggests cold autoagglutinin causing panagglutination.

• Warm saline washes remove nonspecific cold agglutinins from RBC surfaces.

• Autoadsorption at 4 °C removes cold autoantibody from serum, allowing accurate ABO typing.

Summary
Panagglutination across all tests = cold autoagglutinin interference → warm wash and autoadsorb serum to resolve.

What should be done if all forward and reverse ABO results are negative?

A. Perform additional testing, such as typing with Anti-A1 lectin and Anti-A,B

B. Incubate at 22C or 4C to enhance weak expression

C. Repeat the test with new reagents

D. Run an antibody identification panel

B. Incubate at 22 °C or 4 °C to enhance weak expression

• All forward and reverse results negative → likely weak or missing antigen and antibody expression.

• Causes: newborn, elderly, immunodeficient, or hypogammaglobulinemic patients.

• Cool incubation enhances antibody binding and antigen reactivity in weak ABO systems.

Summary
Completely negative ABO reactions → suspect weak expression; repeat at lower temperature to enhance detection.

N-acetyl-D-galactosamine is the immunodominant carbohydrate that reacts with:

A. Arachis hypogaea

B. Salvia sclarea

C. Dolichos biflorus

D. Ulex europeaus

C. Dolichos biflorus

• N-acetyl-D-galactosamine is the immunodominant sugar of the A antigen.

• The lectin from Dolichos biflorus (Anti-A₁ lectin) specifically reacts with A₁ antigen structures containing this sugar.

• Other lectins: Ulex europaeus → Anti-H; Arachis hypogaea → Anti-T; Salvia sclarea → Anti-N.

Summary
N-acetyl-D-galactosamine → A antigen, detected by Dolichos biflorus lectin.

A stem cell transplant recipient was retyped when she was transferred from another hospital. What is the most likely cause of the following results?

Patient Cells: Anti-A, neg; Anti-B, 4+

Patient Serum: A1 cells, neg; B cells, neg

A. Viral infection

B. Alloantibodies

C. Immunodeficiency

D. Autoimmune hemolytic anemia

C. Immunodeficiency

• Forward: Anti-A neg / Anti-B 4+ → group B cells.

• Reverse: A1 cells neg / B cells neg → missing expected isoagglutinins (no anti-A in a B patient).

• HSCT recipients often have hypogammaglobulinemia/immunosuppression → weak/absent ABO antibodies.

Summary
Group B by forward type with absent reverse antibodies in a transplant patient → consistent with immunodeficiency after HSCT.

What reaction would be the same for an A1 and an A2 person?

A. Positive reaction with Anti-A1 lectin

B. Positive reaction with A1 cells

C. Equal reaction with Anti-H

D. Positive reaction with Anti-A,B

D. Positive reaction with Anti-A,B

• Anti-A,B is a broad-spectrum reagent that recognizes the shared A antigen determinant present on all A subgroups.

• Both A₁ and A₂ RBCs express this core A antigen structure, though A₂ expresses it in lower density.

• Anti-A,B combines the specificities of Anti-A and Anti-B, allowing detection of weaker or variant A antigens that Anti-A alone may miss.

• Therefore, both A₁ and A₂ will react positively with Anti-A,B, even if A₂ gives a slightly weaker reaction.

Summary
Anti-A,B detects the common A antigen structure on both A₁ and A₂ cells, producing a positive reaction despite differences in antigen density.

A female patient at 28 weeks' pregnancy yields the following results:

Anti-A 3+

Anti-B 4+

A1 Cells neg

B Cells 1+

O cells 1+

Which of the following could be causing the ABO discrepancy?

A. Hypogammaglobulinemia

B. Alloantibodies in patient serum

C. Acquired B

D. Weak subgroup

B. Alloantibodies in patient serum

• Forward: Anti-A 3+, Anti-B 4+ → AB phenotype (slightly weaker A possible, but AB overall).

• Reverse: A1 cells neg (expected for AB), B cells 1+ and O cells 1+ → unexpected serum reactivity with both B and O cells.

• Reactivity with O cells indicates an antibody not in the ABO system (e.g., anti-H, anti-P1, anti-Le, cold auto), i.e., alloantibody interfering with reverse typing.

Summary
AB forward with unexpected reverse positives (incl. O cells) → points to non-ABO alloantibody in serum, not hypogammaglobulinemia, acquired B, or weak subgroup.

Which condition would most likely be responsible for the following typing results?

Anti-A: neg

Anti-B: neg

A1 cells: neg

B cells: 4+

A. Immunodeficiency

B. Masking of antigens by the presence of massive amounts of antibody

C. Weak or excessive antigen(s)

D. Impossible to determine

C. Weak or excessive antigen(s)

• Pattern: Forward looks O (Anti-A neg, Anti-B neg) but reverse has anti-B only (A1 cells neg, B cells 4+) → suggests group A with weak/undetected A antigen.

• Most consistent with a weak A subgroup (e.g., A₂, A₃, Ax) or weak antigen expression rather than immunodeficiency (which would reduce reverse antibodies).

• Next step: Test with Anti-A,B, A₁ lectin (Dolichos biflorus), extended RT incubation, or adsorb/elute to confirm weak A.

Summary
Forward/reverse mismatch points to weak A antigen expression (not immunodeficiency or blocked antigens).

Which of the following results is most likely discrepant?

Anti-A: neg

Anti-B: 4+

A1 cells: neg

B cells: neg

A. Negative B cells

B. Positive reaction with Anti-B

C. Negative A1 cells

D. No problem with this typing

C. Negative A1 cells

• Forward: Anti-B 4+ / Anti-A neg → group B.

• Reverse (expected for B): A1 cells pos (anti-A present), B cells neg.

• Given A1 cells neg → mismatch; B cells neg is appropriate.

Summary
Group B forward typing requires positive A1 reverse; a negative A1 cell result is the discrepant finding.

A 61 yo male with a history of multiple myeloma underwent stem cell transplantation 3 years ago. The donor was O positive, and the recipient was B positive. The patient is admitted to a community hospital for fatigue and nausea. Typing results reveal the following:

Anti-A: 0

Anti-B: 0

Anti-A,B: 0

Anti-D: 4+

A1 cells: 4+

B cells: 0

How would you report this type?

A. O Pos

B. B Pos

C. A Pos

D. Undetermined

D. Undetermined

• Forward: Anti-A 0, Anti-B 0, Anti-A,B 0 → O cells.

• Reverse: A1 cells 4+, B cells 0 → serum pattern of group B (anti-A present, no anti-B).

• HSCT (O→B) history → possible mixed chimerism/ABO conversion; forward/reverse discordant.

• Report as undetermined; transfuse group O, Rh-compatible RBCs until resolved.

Summary
O-like forward with B-like reverse after HSCT = ABO discrepancy → report undetermined and manage as O RBCs pending clarification.