Blood group of an individual is determined by

Question

UnlockIAS · Accepted Answer

Correct — D, None of the above. Blood group is determined by antigens — glycoproteins and glycolipids — sitting on the surface membrane of the red blood cells, and by the matching antibodies present in the plasma. In the ABO system the antigens are called agglutinogens A and B and the plasma antibodies agglutinins anti-A and anti-B, and the four groups follow mechanically from which of each are present. Group A carries the A antigen and anti-B antibody; group B carries the B antigen and anti-A; group AB carries both antigens and neither antibody, which is why an AB person is a universal recipient; group O carries neither antigen and both antibodies, which with a negative Rh makes O the universal donor. The Rh system runs alongside it on the same cells, its key marker being the D antigen. That mechanism is the answer to the stem, and it is not among the three options — none of them so much as mentions an antigen — so the escape option is correct. Karl Landsteiner, who worked this out at the turn of the twentieth century, received the Nobel Prize in Physiology or Medicine in 1930 'for his discovery of human blood groups'. Each of the three named options fails for its own reason, and each failure is a fact worth holding: shape is identical across groups, haemoglobin is present in everyone's red cells, and white cells play no part in ABO typing at all.

Why the other options are wrong — (a) Every healthy human red blood cell is the same biconcave disc regardless of blood group — the shape is an adaptation for surface area and flexibility, not a marker of identity. When red cells do change shape it signals disease, not group: the crescent cells of sickle cell anaemia, the spheres of hereditary spherocytosis. A person with sickle cell disease can belong to any of the four ABO groups.  (b) Haemoglobin is the oxygen-carrying protein packed inside every red cell of every human being, so it cannot distinguish one person's blood from another's — a property shared by all four groups discriminates none of them. Haemoglobin level is what a blood test measures for anaemia, and it changes with iron status and altitude, not with blood group.  (c) ABO and Rh typing involves red cells only; white blood cells are not part of it. There is a genuine white-cell typing system, and knowing it is what makes this option a trap worth understanding — the human leukocyte antigen system, used to match donors and recipients for organ and bone-marrow transplants. HLA matching and blood grouping answer different clinical questions on different cells.

Concept: The reason blood groups exist at all is immunological. The body tolerates the antigens on its own red cells and manufactures antibodies against the ones it lacks, so transfusing incompatible blood causes the recipient's antibodies to clump the donor's cells — agglutination — with results that can be fatal. That is why the ABO system had to be discovered before transfusion could become safe medicine, and why Landsteiner's work earned a Nobel Prize. The Rh system, identified later, adds a second dimension and a second danger: an Rh-negative mother carrying an Rh-positive foetus can be sensitised to the D antigen and attack the red cells of a subsequent Rh-positive pregnancy, a condition called haemolytic disease of the newborn, which is why anti-D immunoglobulin is given prophylactically. Groups are inherited, with the A and B alleles co-dominant and O recessive, which is why parentage questions can be built on them: an AB parent and an O parent can have A or B children but never an O child.

State PCS 2025 Q70 — Science & Technology PYQ with Answer

State PCS 2025 Q70 — Science & Technology PYQ with Answer