The Red Blood cells of a person with Blood group ‘O’ contains Agglutinogen :

Correct — option (3), Neither A nor B.

An agglutinogen is an antigen on the red blood cell surface. When antibodies in serum bind it, the red cells clump, or agglutinate. In the ABO system the agglutinogens are the A and B antigens.

The NCBI Bookshelf reference Blood Groups and Red Cell Antigens (Laura Dean, 2005) states that people with the common blood type O "express neither the A nor B antigen". So the red cells of a group O person carry Neither A nor B.

The group's name records this. In Landsteiner's grouping, a third group contained red cells that reacted as if they lacked the properties of A and B. It was later called "O" after the German word "Ohne", meaning "without".

What group O blood does carry is in the serum: both anti-A and anti-B antibodies. Antigens are on the cells; antibodies are in the serum.

The idea to remember: group O red cells have no A or B antigen, and group O serum has both anti-A and anti-B; group AB is the reverse.

Why the others are wrong

• (1)
  A and B both — Red cells carrying both A and B antigens belong to blood group AB, not O. Dean's reference describes the AB group as red cells that express both A and B antigens.

  The serum pattern is reversed too: group AB individuals have neither anti-A nor anti-B in their serum, while group O serum has both.

• (2)
  Only B — Red cells with only the B antigen belong to blood group B.

  The immune system forms antibodies against the ABO antigens missing from a person's own red cells. So a group B individual has anti-A antibodies in the serum. A group O person's red cells lack the B antigen as well as the A antigen.

• (4)
  Only A — Red cells with only the A antigen belong to blood group A.

  Following the same rule, a group A individual has anti-B antibodies in the serum. Group A is also split into two main phenotypes, A1 and A2; Dean's reference says A1 red cells express about 5 times more A antigen than A2 red cells. Group O cells carry no A antigen at all.

The ABO blood group is defined by carbohydrate antigens attached to chains that project above the red cell surface. A person's group depends on which of the A and B antigens the red cells carry.

One gene, the ABO locus, has three main alleles: A, B and O. The A and B alleles each make an enzyme that finishes the A or B antigen. The O allele makes an inactive enzyme, leaving the precursor, the H antigen, unmodified.

Dean's reference describes A and B as inherited codominantly over O. A child receives one of the three alleles from each parent, giving six possible genotypes and four blood groups: A, B, AB and O.

Key facts

• Group O red cells express neither the A nor the B antigen (Dean, Blood Groups and Red Cell Antigens, NCBI, 2005).
• Anti-A is found in the serum of people with groups O and B; anti-B in the serum of people with groups O and A.
• Group AB red cells express both A and B antigens, and group AB serum has neither anti-A nor anti-B.
• The O allele encodes an inactive enzyme that leaves the precursor H antigen unmodified; A and B are inherited codominantly over O.
• Karl Landsteiner received the 1930 Nobel Prize in Physiology or Medicine for his discovery of human blood groups.

Common traps

• Antigen and antibody point in opposite directions. Group O red cells carry neither A nor B antigen, but group O serum carries both anti-A and anti-B. Check whether a stem asks about cells or serum.
• Group AB is the mirror image of group O: both antigens on the cells, neither antibody in the serum.
• "O" is not a third antigen. The name comes from the German "Ohne", meaning without; O cells carry the unmodified H precursor instead of A or B.

UnlockIAS will link similar questions from RAS Pre 2021 and 2013 here once those papers are published on this site.

• practice — not a real PYQ

  The serum of a person with blood group 'O' contains:

  • (a)Only anti-A
  • (b)Only anti-B
  • (c)Both anti-A and anti-B
  • (d)Neither anti-A nor anti-B
  Answer(3) — Anti-A is found in the serum of groups O and B, and anti-B in the serum of groups O and A, so group O serum has both. Option (1) fits group B, option (2) fits group A, and option (4) fits group AB.
• practice — not a real PYQ

  A person's red blood cells carry both A and B antigens, and the serum has neither anti-A nor anti-B. The blood group is:

  • (a)O
  • (b)AB
  • (c)A
  • (d)B
  Answer(2) — Group AB red cells express both A and B antigens, and group AB serum has neither antibody. Group O cells carry neither antigen, group A cells carry only A, and group B cells carry only B.