Q.Which blood group is referred to as the 'universal donor', and why?
Concept understanding — Blood Groups
Blood typing exists because every person's red blood cells carry a personal pattern of surface antigens, and mismatching that pattern between donor and recipient triggers a dangerous immune reaction. The ABO system recognises four groups - A, B, AB and O - based on which of two antigens (A and/or B, called agglutinogens) sit on the RBC surface; the plasma simultaneously carries natural antibodies (agglutinins) against whichever antigen a person's own cells lack, so group A blood carries anti-B antibodies, group B carries anti-A, group AB carries neither antibody (since both antigens are already present), and group O carries both anti-A and anti-B antibodies (since neither antigen is present). Transfusing incompatible blood causes the recipient's antibodies to attack the donor's antigen-bearing cells, so compatibility must always be checked. The second system, Rh typing, is based on a single surface protein (the D antigen), present in roughly 80% of people (Rh-positive) and absent in the rest (Rh-negative). Rh mismatch matters especially in pregnancy: if an Rh-negative mother carries an Rh-positive foetus, foetal blood cells that cross into her circulation (mainly at a first delivery) can sensitise her immune system to produce anti-D antibodies, which in a later Rh-positive pregnancy cross the placenta and destroy foetal red cells, causing the potentially fatal condition erythroblastosis foetalis (foetal anaemia and jaundice) - a risk now prevented by giving Rh-negative mothers anti-D antibody immediately after their first Rh-incompatible birth. Because group O, Rh-negative blood carries no A, B or D antigens for a recipient's antibodies to react against, it functions as the universal donor blood type in an emergency where there is no time to test the recipient's own group.
Group O has no A/B antigen on its RBCs, so it will not trigger a recipient's anti-A/anti-B antibody.
Group O; its red cells carry neither antigen A nor antigen B, so they can be given to a recipient of any ABO group without triggering an antigen-antibody reaction.
Step 1. A transfusion reaction occurs when the recipient's antibody attacks an antigen on the donor's red cells.
Step 2. Group O red cells carry neither the A nor the B antigen, so no ABO antibody in any recipient's plasma has a matching antigen to attack.
Group O; its red cells carry neither antigen A nor antigen B, so they can be given to a recipient of any ABO group without triggering an antigen-antibody reaction.
- Confusing universal donor (group O, no antigens) with universal recipient (group AB, no antibodies).
- CBSE 2026Set ANNUAL1 markMCQQ.Erythroblastosis foetalis is due to the destruction of :(a) Foetal WBCs(b) Foetal RBCs(c) Foetus suffers from Minamata(d) Foetus suffers from Atherosclerosis
›Reveal solutionSolution
Erythroblastosis foetalis is caused by maternal anti-Rh antibodies destroying the foetus's Rh-positive red blood cells, an example of Rh incompatibility between mother and foetus.
The Rh factor is an antigen present on the surface of red blood cells in Rh-positive individuals and absent in Rh-negative individuals. If an Rh-negative mother carries an Rh-positive foetus (having inherited the Rh antigen from an Rh-positive father), some foetal red blood cells can leak into the mother's circulation, usually around childbirth or a placental injury, and the mother's immune system responds by producing anti-Rh antibodies. In a subsequent Rh-positive pregnancy, these maternal antibodies can cross the placenta and attack and destroy the foetus's red blood cells (RBCs), a condition called erythroblastosis foetalis (also called haemolytic disease of the newborn). This destruction of foetal RBCs can cause severe anaemia, jaundice, and in serious cases can be fatal to the foetus/newborn — which is why Rh-negative mothers are given an anti-D (Rh immunoglobulin) injection to prevent this immune sensitisation.
✓Final answerThe correct option is (b) Foetal RBCs.
- CBSE 2025Set ANNUAL1 markMCQQ.A person having both antigen A and antigen B on the surface of RBCs belongs to Blood group:(a) AB(b) A(c) O(d) B
›Reveal solutionSolution
A person whose RBCs carry both antigen A and antigen B on their surface belongs to blood group AB.
In the ABO blood grouping system, blood group is determined by the presence or absence of two antigens (agglutinogens), A and B, on the surface of red blood cells:
- Blood group A: only antigen A present.
- Blood group B: only antigen B present.
- Blood group AB: both antigen A and antigen B present.
- Blood group O: neither antigen A nor antigen B present.
Since the question specifies a person having BOTH antigen A and antigen B on their RBCs, that person belongs to blood group AB. (AB individuals are also known as 'universal recipients' since their plasma lacks both anti-A and anti-B antibodies.)
✓Final answer(a) AB.
- CBSE 2022Set ANNUAL1 markMCQQ.Surface antigens on RBCs are called ________.(a) thrombocytes(b) microglia(c) agglutinogens(d) agglutinin
›Reveal solutionSolution
Surface antigens on red blood cells, which determine blood group and can trigger clumping with a mismatched antibody, are called agglutinogens.
Human blood is grouped mainly on the basis of specific glycoprotein/glycolipid molecules present on the plasma membrane of red blood cells. These surface molecules act as antigens because the immune system of a person lacking them will recognise them as foreign; they are specifically termed agglutinogens (e.g., agglutinogen A and agglutinogen B in the ABO system). Circulating in the blood plasma are corresponding antibodies called agglutinins (e.g., anti-A, anti-B) which are NOT present against a person's own agglutinogens (to avoid self-clumping) but ARE present against the agglutinogens that person lacks. When agglutinogen-bearing RBCs of one blood type are mixed with plasma containing the matching agglutinin (as in a mismatched transfusion), the antibody binds the antigen and causes the RBCs to clump together — agglutination. Thrombocytes are platelets involved in clotting, and microglia are immune cells of the nervous system, not RBC surface molecules — both are unrelated to this question.
✓Final answerThe correct option is (c) agglutinogens — these are the surface antigens on RBCs (e.g., antigen A, antigen B) that react with corresponding antibodies (agglutinins) in plasma to cause agglutination (clumping).
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