The ABO Blood Group System is the most clinically significant blood group system in transfusion medicine. Correct identification of ABO blood types is essential to prevent fatal transfusion reactions and ensure compatibility between donor and recipient blood.
In this section, students and professionals will explore the genetic basis, antigenic structures, and laboratory testing procedures that define the ABO system — the cornerstone of all blood bank practices.
Topics Covered in This Chapter
- History and importance of the ABO system
- Antigens A, B, AB, and O – structure and biochemical nature
- Genetics and inheritance patterns of the ABO system
- Forward (cell) and reverse (serum) grouping
- Subgroups of A (A₁, A₂, etc.) and their identification
- Discrepancies in ABO testing and troubleshooting techniques
- Clinical significance of ABO incompatibility in transfusion and pregnancy
Why It Matters
The ABO system is the foundation of blood transfusion safety. A simple error in ABO grouping can lead to severe hemolytic reactions and patient harm. Understanding the molecular and serologic principles behind ABO testing is critical for every Medical Laboratory Scientist and Blood Bank professional.
Learning Outcomes
After studying this chapter, learners should be able to:
- Describe the genetics and antigenic structure of the ABO system.
- Perform and interpret forward and reverse grouping accurately.
- Recognize subgroups of A and identify testing discrepancies.
- Explain the clinical implications of ABO incompatibility.
60 MCQs (4361 – 4420):
- The ABO blood group system is based on the presence or absence of which substances?
a) Proteins on platelets
b) Antigens on red blood cells
c) Antibodies in plasma only
d) Enzymes on leukocytes - The antigens of the ABO system are primarily composed of:
a) Proteins
b) Glycolipids and glycoproteins
c) Lipids
d) Enzymes - The “H” antigen serves as a:
a) Precursor structure for A and B antigens
b) Direct antigen in the Rh system
c) Coating protein for platelets
d) Factor in complement activation - The gene responsible for the “H” antigen is located on which chromosome?
a) 1
b) 9
c) 19
d) 22 - The absence of H antigen on red cells characterizes which phenotype?
a) Bombay (Oh)
b) A₂
c) B₃
d) O positive - In the ABO system, the A and B antigens differ by:
a) The addition of different terminal sugars
b) The number of amino acids
c) The protein structure of hemoglobin
d) The antibody type - The enzyme that adds N-acetylgalactosamine to form A antigen is known as:
a) Transferase A
b) Galactosidase
c) Glycosidase
d) Beta-glucosidase - The most common ABO phenotype worldwide is:
a) AB
b) A
c) B
d) O - ABO antibodies are primarily of which immunoglobulin class?
a) IgG
b) IgM
c) IgA
d) IgE - ABO antibodies usually appear in the serum:
a) At birth
b) Within the first few weeks of life
c) During the fetal stage
d) Only after transfusion - Which method is used to detect ABO antigens on red cells?
a) Reverse grouping
b) Forward grouping
c) Coagulation test
d) Antiglobulin test - Which method detects ABO antibodies in serum or plasma?
a) Forward grouping
b) Reverse grouping
c) Crossmatching
d) DAT - A person with genotype AO will have which phenotype?
a) A
b) O
c) B
d) AB - A discrepancy between forward and reverse grouping may be caused by:
a) Incorrect centrifugation speed
b) Cold agglutinins or weak antibodies
c) Excess plasma volume
d) Use of outdated reagents - Which of the following is a subgroup of A with weaker antigen expression?
a) A₁
b) A₂
c) B₃
d) AB - Which lectin is used to differentiate between A₁ and A₂ cells?
a) Dolichos biflorus
b) Ulex europaeus
c) Phaseolus vulgaris
d) Vicia graminea - The A₂ phenotype differs from A₁ because A₂ cells:
a) Possess fewer antigen sites
b) Lack the H antigen
c) Are strongly reactive with anti-A₁
d) Have more A antigen sites - The A₁ antigen reacts with:
a) Anti-A and anti-A₁
b) Anti-B and anti-A
c) Anti-H only
d) Anti-A₂ and anti-B - Which reagent is used to identify H antigen expression?
a) Dolichos biflorus lectin
b) Ulex europaeus lectin
c) A₁ cells
d) Anti-D reagent - Individuals with the Bombay phenotype will type as:
a) O group in forward grouping but have anti-H in serum
b) AB group with weak reactivity
c) B group with anti-A
d) A₂ group with no anti-H - Mixed-field agglutination in ABO testing may indicate:
a) Polyagglutination
b) Recent transfusion or chimerism
c) Weak subgroup of B
d) Clerical error only - Which of the following can cause weak or missing antibodies in reverse grouping?
a) Newborn age or immunodeficiency
b) Cold agglutinins
c) Excess H antigen
d) Overcentrifugation - In ABO typing, hemolysis after adding anti-A serum indicates:
a) Strong antibody presence
b) Positive antigen–antibody reaction
c) Test invalidity
d) Protein degradation - Which type of error would result in a false-positive agglutination?
a) Dirty glassware or contaminated reagents
b) Low incubation temperature
c) Incomplete washing
d) Excess saline dilution - The A₃ subgroup is characterized by:
a) Weak mixed-field reaction with anti-A
b) No reaction with anti-A or anti-B
c) Strong reaction with both anti-A and anti-B
d) Agglutination with anti-H only - Reverse grouping discrepancies may be due to:
a) Cold or unexpected antibodies
b) Lack of H antigen
c) Weak D antigen
d) Bacterial contamination - Which antigen is present in group O individuals?
a) A antigen
b) B antigen
c) H antigen
d) No antigen - The presence of anti-A and anti-B antibodies in serum indicates which blood group?
a) Group A
b) Group B
c) Group AB
d) Group O - Group AB individuals can receive blood from:
a) Only group O
b) All ABO groups (universal recipient)
c) Group A and B only
d) Group O and AB only - The phenomenon of “acquired B” antigen can occur in:
a) Certain bacterial infections altering red cell membranes
b) Iron deficiency anemia
c) Vitamin B12 deficiency
d) Plasma protein disorders - A blood donor has the genotype: hh, AB. Using anti-A and anti-B antisera, the donor’s red cells will type as group:
a) A
b) B
c) O
d) AB - What do the O<sub>h</sub> (Classical Bombay), group O, and Lu(a-b-) phenotypes have in common?
a) Result from inheritance of identical sex-linked dominant genes
b) Result from inheritance of identical sex-linked recessive genes
c) Result from inheritance of identical autosomal dominant genes
d) Result from inheritance of identical autosomal recessive genes - H antigen is the immunodominant precursor for A and B antigens. In which individuals is H antigen found in its highest concentration on red blood cells?
a) Group A<sub>1</sub>
b) Group A<sub>2</sub>
c) Group B
d) Group O - The genetic product of the H gene is an enzyme that confers H antigen specificity by adding which immunodominant sugar?
a) N-acetylgalactosamine
b) Galactose
c) Fucose
d) N-acetylglucosamine - A patient’s red blood cells type as 4+ with anti-A, 0 with anti-B, and 4+ with anti-A,B. The serum reacts 0 with A<sub>1</sub> cells and 4+ with B cells. What is the patient’s ABO group?
a) A
b) B
c) O
d) AB - An individual with the Bombay phenotype (Oh):
a) Has anti-A and anti-B in the serum
b) Has anti-A, anti-B, and anti-H in the serum
c) Has no ABO antibodies in the serum
d) Has anti-H in the serum, but no anti-A or anti-B - Which of the following ABO subgroups is characterized by mixed-field agglutination with anti-A and anti-A,B?
a) A1
b) A2
c) A3
d) Ax - A patient is a subgroup of A (Asub), Rh-positive with Anti-A1 in their serum. How many random group A units would you have to screen to find one unit that is compatible?
a) 5
b) 10
c) 15
d) 20 - The immunodominant sugar responsible for blood group A specificity is:
a) Fucose
b) Galactose
c) N-acetylgalactosamine
d) N-acetylglucosamine - Which of the following forward and reverse ABO typing results is typical for an Ax subgroup?
a) Cells: 4+ with anti-A, 0 with anti-B; Serum: 0 with A1 cells, 4+ with B cells
b) Cells: 0 with anti-A, 4+ with anti-B; Serum: 4+ with A1 cells, 0 with B cells
c) Cells: Weak with anti-A, 0 with anti-B; Serum: 4+ with A1 cells, 4+ with B cells
d) Cells: Weak with anti-A, 0 with anti-B; Serum: 0 with A1 cells, 4+ with B cells - In what patient population are ABO antibodies not expected to be present?
a) A 30-year-old GI bleed patient
b) A 2-year-old pre-surgical patient
c) A 16-year-old surgery patient
d) A 45-year-old pregnant female - A patient is group A2B, Rh-positive and has an antiglobulin-reacting anti-A1 in his serum. If A2B Red Blood Cells are not available, which of the following blood types is the first choice for crossmatching?
a) B, Rh-positive
b) B, Rh-negative
c) A1B, Rh-positive
d) O, Rh-negative - The purpose of testing with anti-A,B is to help detect:
a) Anti-A1
b) Anti-A2
c) Subgroups of A
d) Subgroups of O - A patient’s ABO typing shows 4+ reactions with anti-A and anti-B, but no reaction with A1 or B cells in the reverse type. What is the most likely cause?
a) Acquired B phenomenon
b) Cold autoantibody
c) Hypogammaglobulinemia
d) Ax subgroup - ABO hemolytic disease of the newborn:
a) Usually requires an exchange transfusion
b) Most often occurs in first-born children
c) Frequently results in stillbirth
d) Is usually seen only in the newborn of group O mothers - A patient with multiple myeloma has rouleaux formation interfering with reverse typing. What technique can resolve this discrepancy?
a) Cold autoadsorption
b) Saline replacement
c) Enzyme treatment of patient cells
d) Treatment of serum with dithiothreitol (DTT) - Which of the following lectins is used to differentiate between A<sub>1</sub> and A2 subgroups?
a) Ulex europaeus
b) Arachis hypogaea
c) Dolichos biflorus
d) Vicia graminea - A group A, Rh-positive infant of a group O, Rh-positive mother has a weakly positive direct antiglobulin test and a moderately elevated bilirubin 12 hours after birth. The most likely cause is:
a) ABO incompatibility
b) Rh incompatibility
c) Blood group incompatibility due to an antibody to a low-frequency antigen
d) Neonatal jaundice not associated with blood group - In the “acquired B” phenomenon, a group A patient’s red cells appear to react with anti-B due to:
a) Bacterial contamination
b) Deacetylating enzymes altering the A antigen
c) A genetic mutation
d) Transfusion of group B plasma - For a patient with the Bombay phenotype (Oh), what is the only safe source of Red Blood Cells for transfusion?
a) Group O, H-negative
b) Group O, H-positive
c) Another Bombay phenotype donor
d) Autologous donation only - Which of the following genes must be inherited for an individual to be a secretor of ABH antigens in their saliva?
a) H only
b) Se only
c) H and Se
d) Le and Se - What antigens would be found in the saliva of an individual with the genotype Sese Lele AO HH?
a) A, H
b) Leb, A, H
c) Lea, Leb, A, H
d) Lea - ABO-hemolytic disease of the newborn differs from Rh HDFN in that:
a) Rh HDFN is clinically more severe than ABO HDFN
b) The direct antiglobulin test is weaker in Rh HDFN than ABO HDFN
c) Rh HDFN occurs in the first pregnancy
d) The mother’s antibody screen is positive in ABO HDFN - A patient’s forward typing shows 3+ with anti-A and 4+ with anti-B. Reverse typing shows 2+ with A1 cells and 0 with B cells. What is a likely explanation?
a) The patient is group AB with a cold alloantibody
b) The patient is group B with rouleaux
c) The patient is group A with an acquired B antigen
d) The patient is a normal group AB - When preparing units for an exchange transfusion for an infant with ABO HDFN, which of the following units should be selected if the infant is group A and the mother is group O?
a) Group A, Rh-positive Red Blood Cells
b) Group O, Rh-positive Red Blood Cells
c) Group O, Rh-negative Red Blood Cells suspended in group AB plasma
d) Group A, Rh-positive Red Blood Cells suspended in group AB plasma - The absence of which gene leads to the classical Bombay (Oh) phenotype?
a) The A gene
b) The B gene
c) The O gene
d) The H gene - A patient’s sample shows agglutination with anti-A and anti-B, but no reaction with A1 cells and weak reaction with B cells. The direct antiglobulin test is negative. This discrepancy is most likely due to:
a) A cold autoantibody
b) Hypogammaglobulinemia
c) An ABO subgroup with an unexpected antibody
d) Rouleaux - In a group O individual with Le and Se genes, what ABH and Le antigens are present in their secretions?
a) Lea, Leb
b) Lea, LeH
c) Lea, H
d) Leb, H - Which of the following is a characteristic of the A2 phenotype?
a) Cells react weakly with anti-A,B
b) Serum always contains anti-A1
c) Cells do not react with Dolichos biflorus lectin
d) It is the most common A subgroup - The fundamental principle of ABO typing involves:
a) Detecting the presence of antigens on white blood cells and corresponding antibodies in serum
b) Detecting the presence of antigens on red blood cells and the corresponding antibodies in serum
c) Detecting molecular sequences on DNA
d) Detecting the presence of enzymes in the plasma
📌 How to Use This Practice Set
- Answer each question before checking the key.
- Focus on why the correct answer is right and the others are wrong.
- Use this set as timed practice to simulate the real exam environment.
Answer Key
Answer Key:
- b) Antigens on red blood cells
- b) Glycolipids and glycoproteins
- a) Precursor structure for A and B antigens
- c) 19
- a) Bombay (Oh)
- a) The addition of different terminal sugars
- a) Transferase A
- d) O
- b) IgM
- b) Within the first few weeks of life
- b) Forward grouping
- b) Reverse grouping
- a) A
- b) Cold agglutinins or weak antibodies
- b) A₂
- a) Dolichos biflorus
- a) Possess fewer antigen sites
- a) Anti-A and anti-A₁
- b) Ulex europaeus lectin
- a) O group in forward grouping but have anti-H in serum
- b) Recent transfusion or chimerism
- a) Newborn age or immunodeficiency
- b) Positive antigen–antibody reaction
- a) Dirty glassware or contaminated reagents
- a) Weak mixed-field reaction with anti-A
- a) Cold or unexpected antibodies
- c) H antigen
- d) Group O
- b) All ABO groups (universal recipient)
- a) Certain bacterial infections altering red cell membranes
- c) O
- d) Result from inheritance of identical autosomal recessive genes
- d) Group O
- c) Fucose
- a) A
- b) Has anti-A, anti-B, and anti-H in the serum
- c) A3
- a) 5
- c) N-acetylgalactosamine
- d) Cells: Weak with anti-A, 0 with anti-B; Serum: 0 with A1 cells, 4+ with B cells
- b) A 2-year-old pre-surgical patient
- a) B, Rh-positive
- c) Subgroups of A
- c) Hypogammaglobulinemia
- d) Is usually seen only in the newborn of group O mothers
- b) Saline replacement
- c) Dolichos biflorus
- a) ABO incompatibility
- b) Deacetylating enzymes altering the A antigen
- c) Another Bombay phenotype donor
- c) H and Se
- b) Leb, A, H
- a) Rh HDFN is clinically more severe than ABO HDFN
- c) The patient is group A with an acquired B antigen
- c) Group O, Rh-negative Red Blood Cells suspended in group AB plasma
- d) The H gene
- c) An ABO subgroup with an unexpected antibody
- d) Leb, H
- a) Cells react weakly with anti-A,B
- b) Detecting the presence of antigens on red blood cells and the corresponding antibodies in serum
Top 8 Medical Laboratory Scientist (MLS) Exams:
Top 8 Medical Laboratory Scientist (MLS) Exams that are recognized globally and can help professionals validate their credentials and enhance their career opportunities:
1. ASCP – American Society for Clinical Pathology (USA)
- Exam Name: MLS(ASCP)
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