What are the four human blood types in ABO blood group typing?

Q: What is the answer to CDS (II) 2025 GK Q43?

B. Correct — B, A, B, AB and O. What is being typed is the antigen carried on the surface of a person's red cells. Carry the A antigen and you are group A; carry B and you are group B; carry both and you are AB; carry neither and you are O. That is four possibilities and no more, and the plasma completes the pattern by carrying antibodies against whichever antigen is absent — group A plasma has anti-B, group B has anti-A, group O has both, and group AB has neither. This is why group O red cells can go to anyone, since there is no antigen for a recipient's antibodies to attack, while a person of group AB can receive from anyone, since there is no antibody in their plasma to do the attacking. The reason the other options look plausible is that they mix in genotype labels. Three alleles produce six genotypes — AA, AO, BB, BO, AB and OO — but A is dominant over O and B is dominant over O, so AA and AO both show as group A, and BB and BO both show as group B. Six genotypes, four blood types. Karl Landsteiner identified the system in 1901 and was awarded the Nobel Prize in Physiology or Medicine for it in 1930. (Provisional — pending the official UPSC key.)

(a)AO, BO, A and B
(b)A, B, AB and O
(c)A, AB, BO and O
(d)AB, AO, B and O

Answer

Why

Correct — B, A, B, AB and O. What is being typed is the antigen carried on the surface of a person's red cells. Carry the A antigen and you are group A; carry B and you are group B; carry both and you are AB; carry neither and you are O. That is four possibilities and no more, and the plasma completes the pattern by carrying antibodies against whichever antigen is absent — group A plasma has anti-B, group B has anti-A, group O has both, and group AB has neither. This is why group O red cells can go to anyone, since there is no antigen for a recipient's antibodies to attack, while a person of group AB can receive from anyone, since there is no antibody in their plasma to do the attacking. The reason the other options look plausible is that they mix in genotype labels. Three alleles produce six genotypes — AA, AO, BB, BO, AB and OO — but A is dominant over O and B is dominant over O, so AA and AO both show as group A, and BB and BO both show as group B. Six genotypes, four blood types. Karl Landsteiner identified the system in 1901 and was awarded the Nobel Prize in Physiology or Medicine for it in 1930.

Why the others are wrong

(a)AO, BO, A and B — AO and BO are genotypes, not blood types. Because A and B are each dominant over O, a person of genotype AO tests as group A and one of genotype BO tests as group B. The list also omits AB and O entirely.
(c)A, AB, BO and O — Correct on A, AB and O, but BO is again a genotype rather than a type, and the real fourth group, B, has been left out. A person of genotype BO simply belongs to group B.
(d)AB, AO, B and O — Includes AB, B and O correctly but replaces group A with the genotype AO. The four types are named by the antigens present, and 'AO' names a pair of alleles instead.

Concept

The ABO system classifies blood by which of two antigens, A and B, sit on the surface of the red cells. Four combinations are possible, giving groups A, B, AB and O. Plasma carries naturally occurring antibodies against the antigens the person does not have, so mixing incompatible blood makes the cells clump together and is dangerous. Inheritance is by three alleles at one locus, of which the A and B alleles are each dominant over the O allele and co-dominant with each other, giving six genotypes and four phenotypes. The Rh system, based on the D antigen, is separate, and is what adds the positive or negative suffix to a blood group.

The trap here is a genuine biology confusion dressed up as a memory test: three of the four option lists slip a genotype into a list of phenotypes. If a label contains an O alongside an A or a B, it is describing the pair of alleles a person inherited, not the group their blood tests as, because the O allele produces no antigen and is masked by either of the others. The reliable check is to count antigens rather than letters. Two antigens, each present or absent, give four cells in the table, so any correct list must have exactly four entries and must include AB, which is both present, and O, which is neither. Only option (b) meets both conditions.

Key facts

The four ABO blood types are A, B, AB and O, defined by which of the A and B antigens are present on the red cells.
Plasma carries antibodies against the absent antigen: group A has anti-B, group B has anti-A, group O has both, group AB has neither.
Group O is the universal red-cell donor and group AB the universal recipient, for exactly that reason.
Three alleles produce six genotypes — AA, AO, BB, BO, AB and OO — but only four blood types, because A and B are each dominant over O.
Karl Landsteiner discovered the ABO system in 1901 and received the Nobel Prize in Physiology or Medicine for it in 1930.

Study next

Antigens and antibodies, and why transfusing incompatible blood makes red cells clumpUniversal donor and universal recipient, and the limits of both ideas in practiceInheritance of blood groups — multiple alleles, dominance and co-dominanceThe Rh factor, and Rh incompatibility between a mother and her unborn childBlood composition — plasma, red cells, white cells and platelets, and what each does

Common traps

Treating AO or BO as blood types; they are genotypes, and both show up as group A and group B respectively.
Forgetting group AB, the one in which both antigens are present and neither antibody.
Mixing the Rh system into the ABO count; positive and negative are a separate classification laid on top of the four groups.

Usually as a direct recall of the four groups, or through the universal donor and recipient idea, or as a genetics problem asking which blood groups the children of a given couple could have.

Related PYQs

UPSC_1995_GS1_Q1201995Open & attempt →

A person with ‘AB’ blood group is sometimes called a universal recipient because of the

(a) lack of antigen in his blood
(b) lack of antibodies in his blood
(c) lack of both antigens and antibodies in his blood
(d) presence of antibodies in his blood

Answer(b) lack of antibodies in his blood

The antigen and antibody logic that produces the four groups. Group AB carries both antigens and therefore no anti-A or anti-B antibody, which is exactly what makes it the universal recipient.

Practice

practice — not a real PYQ
A person of blood group O is described as a universal donor of red cells because their red cells
- (a)carry both A and B antigens
- (b)carry neither A nor B antigen
- (c)carry no antibodies
- (d)carry the Rh antigen
Answer(b) carry neither A nor B antigen — with no antigen for the recipient's antibodies to attack, group O red cells can be given to a person of any ABO group.
practice — not a real PYQ
How many genotypes are possible in the ABO blood group system?
- (a)Three
- (b)Four
- (c)Six
- (d)Eight
Answer(c) Six — AA, AO, BB, BO, AB and OO. These collapse into only four blood types, because A and B are each dominant over O.