Q.Which of the following statements is incorrect?
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Blood Groups: ABO and Rh Systems
Every red blood cell carries identity markers called antigens on its surface. Your immune system uses these markers to tell "self" from "foreign." If a foreign antigen shows up in your bloodstream, your plasma antibodies attack it. Blood grouping is simply a classification of which antigens a person's red cells carry, and the two systems that matter most clinically are the ABO system and the Rh system.
The ABO System
Two antigens define this system: A and B. Depending on which ones a person inherits, they fall into one of four groups.
| Blood Group | Antigen on RBCs | Antibody in Plasma |
|---|---|---|
| A | A | Anti-B |
| B | B | Anti-A |
| AB | A and B | None |
| O | None | Anti-A and Anti-B |
The governing rule is that a person never carries an antibody against their own antigen — that would mean the immune system attacking its own blood cells. So a person with antigen A makes anti-B antibodies (ready to attack any B cells that enter), and a person with no antigens at all (group O) makes both anti-A and anti-B.
Golden rule of transfusion: a recipient can only accept red cells that do not carry an antigen the recipient's own antibodies would attack.
Working Out Transfusion Compatibility
- Group A (anti-B present) cannot accept B or AB blood, but can accept A or O.
- Group B (anti-A present) cannot accept A or AB blood, but can accept B or O.
- Group AB has no antibodies at all, so it can accept blood from any ABO group — the universal recipient.
- Group O carries both antibodies, so it can only accept O blood — but because its red cells carry no antigen for anyone else's antibodies to attack, it can donate to any group — the universal donor.
Picture the recipient's antibodies as locks and the donor's antigens as keys. If a donor key fits a recipient lock, the transfusion is rejected.
The Rh System
The Rh system centres on a single important antigen, the Rh (D) factor. A person is either:
- Rh-positive (Rh+) — carries the Rh antigen on the red cells, or
- Rh-negative (Rh-) — does not carry it.
Unlike the ABO system, an Rh-negative person is not born with anti-Rh antibodies. Those antibodies only appear after the person's immune system has actually been exposed to Rh-positive blood, through a transfusion or a pregnancy.
- An Rh-negative person should receive Rh-negative blood.
- An Rh-positive person can safely receive either Rh-positive or Rh-negative blood.
The real danger is sensitisation: once an Rh-negative person has built anti-Rh antibodies from a first exposure, a later transfusion (or pregnancy) involving Rh-positive blood can trigger a severe immune reaction.
Combining Both Systems …
Blood groups are designated on the basis of the antigens present on the surface of the red blood cells (the A and B antigens for the ABO system), not on the basis of the antibodies present in the plasma — the antibodies are simply the consequence of which antigens are absent, not the defining basis of the grouping. The other three statements are all accurate: a person of blood group O does carry both anti-A and anti-B antibodies in their plasma, a person of group B cannot donate to a person of group A (since the recipient's anti-B antibodies would a …
ABO blood groups are named after the antigens carried on the red blood cell surface, not after the antibodies found in the plasma, which makes statement (C) the incorrect one among the four.
The ABO system sorts blood according to the presence or absence of two surface antigens on the red blood cells, called A and B — this antigen pattern is what actually defines a person's blood group (A, B, AB or O). The antibodies present in the plasma are matched to this antigen pattern (a person's plasma never contains an antibody against an antigen present on their own cells), but they are a consequence of the grouping, not the basis on which the grouping itself is designated. …
Method: Finding the Incorrect Statement Among Several True-Sounding Ones
Use this approach for any MCQ asking which of four statements is false.
Steps
Step 1: Build the underlying reference table before reading the options
For ABO blood groups, this means the antigen-antibody table: which antigen sits on the red cell surface and which antibody circulates in the plasma for each group.
Step 2: Check each statement against the table individually, marking TRUE or FALSE
Do this for all four statements methodically — don't stop as soon as one "feels off," since a feeling is not verification.
Step 3: Pay special attention to statements about HOW something is DEFINED or CLASSIFIED …
- AP EAPCET 2024Set ap-2024-05-16-FN1 markMCQQ.Identify the incorrect statements regarding blood groups I. The mother with Rh⁻ blood group develops passive immunity on administering IgM antibodies. II. Isoagglutinogens are sugar polymers bound to fatty acids on the RBC membrane III. The alleles for Rh factor are located on three different loci on chromosome pair 9. IV. The expression of ABO blood types in man is by three alleles of a single gene (A) II, IV (B) I, II (C) I, III (D) II, III
›Reveal solutionSolution
Statement I wrongly names IgM instead of IgG for Rh prophylaxis, and statement III wrongly places the Rh gene on chromosome 9 (that's the ABO locus) instead of chromosome 1 — both are factually incorrect.
Concept and Intuition
Blood group genetics involves specific chromosomal loci and specific antibody classes:
- Rh-negative mothers carrying an Rh-positive fetus are given anti-Rh IgG antibody (which crosses the placenta) to prevent active maternal immunization — not IgM (IgM does not cross the placenta and is not used this way).
- Isoagglutinogens (ABO antigens) are indeed glycolipids/glycoproteins on the RBC membrane — a correct general description.
- The Rh blood group gene(s) are located on chromosome 1, not chromosome 9. Chromosome 9 actually carries the ABO gene locus.
- ABO blood types are expressed via three alleles (IA, IB, i) of a single gene — a textbook example of multiple allelism — correct.
Step-by-Step Solution
- I: Claims IgM is administered — actually IgG (anti-D) is used clinically for Rh prophylaxis → incorrect.
- II: Isoagglutinogens as sugar polymers bound to fatty acids (glycolipid antigens) on RBC membrane → correct description. …
- AP EAPCET 2023Set ap-2023-05-22-AN1 markMCQQ.The universal donor blood group is (A) O+ (B) O− (C) AB+ (D) AB−
›Reveal solutionSolution
O− red cells carry no A, B, or Rh(D) surface antigens, so they can be safely transfused into recipients of any ABO/Rh type, making O− the universal donor.
Concept and Intuition
Blood group compatibility depends on surface antigens (A, B, Rh/D) on red blood cells and the corresponding antibodies in recipient plasma. A donor's cells are "universal" only if they present no antigens that could be attacked by any recipient's antibodies.
Step-by-Step Solution
- O+ red cells lack A and B antigens but do carry the Rh(D) antigen, so they cannot be safely given to Rh-negative recipients — ruling out option (A).
- O− red cells lack A, B, and Rh(D) antigens entirely — they are compatible with any ABO/Rh recipient type. …
- AP EAPCET 2022Set ap-2022-07-11-AN1 markMCQQ.ABO blood group system was proposed by (A) Mendel (B) Morgan (C) Bernstein (D) Karl Landsteiner
›Reveal solutionSolution
Tests a factual/historical point in genetics: the ABO blood group system was discovered by Karl Landsteiner.
Concept and Intuition
Karl Landsteiner, an Austrian biologist and physician, identified the ABO blood groups by observing agglutination reactions between different individuals' blood samples, laying the foundation for safe blood transfusion.
Step-by-Step Solution
- Mendel (option A) is known for the laws of inheritance from pea plant experiments, not blood groups.
- Morgan (option B) is known for chromosomal theory of inheritance (Drosophila work), not ABO discovery.
- Bernstein (option C) later worked out the genetic (multiple-allele) basis of ABO inheritance mathematically, but did not originally discover the blood groups. …
- AP EAPCET 2022Set ap-2022-07-12-AN1 markMCQQ.Which of the following is incorrect? (A) Karl Landsteiner and A.S Weiner discovered Rh antigen (B) Rh factor is called D antigen (C) Person having D antigen is called Rh−ve (D) Person having D antigen is called Rh+ve
›Reveal solutionSolution
Presence of the D antigen defines Rh-positive status, not Rh-negative — so the statement claiming D-antigen-positive individuals are "Rh-ve" is the incorrect one.
Concept and Intuition
The Rh blood group system, named after the Rhesus monkey in which it was first studied, is defined by the presence or absence of the Rh (D) antigen on red blood cell surfaces:
- Individuals with the D antigen present are classified as Rh-positive (Rh+ve).
- Individuals lacking the D antigen are classified as Rh-negative (Rh-ve). This system was discovered by Karl Landsteiner and Alexander S. Wiener. It underlies important clinical phenomena such as hemolytic disease of the newborn (erythroblastosis fetalis) when an Rh-negative mother carries an Rh-positive fetus.
Step-by-Step Solution
- Verify (A): Landsteiner and Wiener did discover the Rh factor — true statement.
- Verify (B): The Rh factor is indeed called the D antigen — true statement.
- Verify (D): A person having the D antigen is correctly called Rh-positive (Rh+ve) — true statement. …
- AP EAPCET 2022Set ap-2022-07-12-AN1 markMCQQ.Which of the following genotype produces Rh negative phenotype? (A) Cdd (B) CdC (C) CDE (D) CdE
›Reveal solutionSolution
Only the genotype containing dd (Cdd) yields an Rh-negative phenotype.
Concept and Intuition
The Rh system is controlled by the D antigen, whose allele D is dominant over d. Presence of at least one D allele gives Rh positive; only the homozygous recessive dd condition, lacking the D antigen, gives Rh negative.
Step-by-Step Solution
- Rh negative requires absence of the D antigen — genotype must be homozygous dd.
- (A) Cdd contains two d alleles (dd) and no D — Rh negative.
- (C) CDE contains D — Rh positive.
- (B) CdC and (D) CdE do not carry the homozygous dd combination needed for the Rh-negative phenotype.
- Hence (A) Cdd produces the Rh-negative phenotype.
Common Mistakes
- Assuming any genotype with a single 'd' is Rh negative; two d alleles are required. …
- AP EAPCET 2021Set ap-2021-09-06-FN1 markMCQQ.A certain road accident patient with unknown blood group needs immediate blood transfusion. His own doctor friend at once offers his blood. What was the blood group of the donor? (A) Blood group AB (B) Blood group B (C) Blood group O (D) Blood group A
›Reveal solutionSolution
With the recipient's blood group unknown, only a universal donor (blood group O) can be safely used immediately.
Concept and Intuition
ABO blood-group compatibility depends on whether the donor's RBC surface antigens (A and/or B) will react with antibodies already present in the recipient's plasma. Blood group O red cells carry NEITHER A nor B antigen, so they cannot be agglutinated by anti-A or anti-B antibodies in a recipient of any ABO group — making O the 'universal donor' for emergency transfusion when the recipient's group is not known.
Step-by-Step Solution
- The scenario states the patient's blood group is UNKNOWN and transfusion is needed immediately — no time to test/cross-match.
- Recall ABO compatibility rules: group AB is the universal recipient (no anti-A/anti-B antibodies), and group O is the universal donor (no A/B antigens on RBCs). …
- AP EAPCET 2021Set ap-2021-09-06-FN1 markMCQQ.Match the following and choose correct combination related to blood groups of Man? List I | List IIi) Sensitized Mother | a) Number of genotypes for multiple allelesii) HDNB | b) With memory cells for D – Antigeniii) Isoagglutinogen | c) Antibody of other individual, but same speciesiv) Isoagglutinin | d) Antigen of other individual, but same speciesv) 2n(n+1) | e) IgG antibody (A) (i – d), (ii – c), (iii – e), (iv – b), (v – a) (B) (i – c), (ii – d), (iii – e), (iv – b), (v – a) (C) (i – d), (ii – e), (iii – a), (iv – b), (v – c) (D) (i – b), (ii – e), (iii – d), (iv – c), (v – a)
›Reveal solutionSolution
Matching blood-group-related terms to their correct descriptions gives (i–b), (ii–e), (iii–d), (iv–c), (v–a).
Concept and Intuition
Rh incompatibility arises when an Rh-negative mother is exposed to Rh-positive (D-antigen bearing) fetal blood, usually during a first delivery; this 'sensitizes' her immune system, producing long-lived memory cells against the D antigen. In a later Rh-positive pregnancy, these memory cells rapidly produce IgG anti-D antibodies (small enough to cross the placenta), causing Haemolytic Disease of the Newborn (Erythroblastosis foetalis). Separately, 'isoagglutinogen' and 'isoagglutinin' are general genetics/immunology terms: an antigen belonging to another member of the same species is an isoagglutinogen, while the corresponding antibody against it in another individual is an isoagglutinin. Finally, for a gene with n multiple alleles (like the ABO system, n=3), the number of possible genotype combinations is 2n(n+1).
Step-by-Step Solution
- Sensitized mother (i): has memory cells against the D-antigen → (b).
- HDNB (ii): mediated by IgG antibody crossing the placenta → (e).
- Isoagglutinogen (iii): antigen of another individual, same species → (d). …
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