Biology · Principles of Inheritance and Variation · NEET
Two things must match: the antigen (sugar) on the donor's red blood cells and the antibody in the recipient's plasma. If the recipient's plasma has an antibody against the donor's antigen, the blood clumps (agglutinates). So the danger comes from the DONOR'S antigen meeting the RECIPIENT'S antibody. If they never meet a matching pair, the transfusion is safe.
O blood group has genotype ii. In NCERT terms, allele i makes no sugar, so O red blood cells carry NEITHER the A antigen NOR the B antigen. When O blood is given to any person, there is no A or B sugar on the donor cells for that person's antibodies to attack. Nothing to offend, so O can donate to A, B, AB and O. This is why O is called the universal donor.
AB blood group has genotype IA IB. Both IA and IB express their sugars (co-dominance), so AB red cells carry BOTH A and B antigens. Because the body already has both sugars, its plasma makes NEITHER anti-A NOR anti-B antibodies. With no antibodies to react against incoming A or B antigens, an AB person can receive A, B, AB or O blood safely. That is why AB is the universal recipient.
Both, but from different sides. Universal DONOR (O) is defined by the ABSENCE of A and B ANTIGENS on the donor red blood cells. Universal RECIPIENT (AB) is defined by the ABSENCE of anti-A and anti-B ANTIBODIES in the recipient plasma. NEET loves to flip these two, so read the question carefully: donor = antigen story, recipient = antibody story.
O plasma has BOTH anti-A and anti-B antibodies (because O cells have no A or B antigen, so the body treats A and B as foreign). If you give A, B or AB blood to an O person, these antibodies attack the A or B antigens on the donor cells and the blood clumps. So O can give to all but can only safely receive from O. It is a great donor but a poor recipient.
Co-dominance means both IA and IB show their effect fully in the heterozygote, so AB cells display both A and B sugars. Because both sugars are seen as 'self', the immune system does not make anti-A or anti-B. No antibodies in plasma is the exact reason AB can accept any group. This is why the ABO system is the classic NCERT example of co-dominance.
Persons with 'AB' blood group are called as 'Universal recipients'. This is due to:
Which of the following characteristics represent 'Inheritance of blood groups' in humans? a. Dominance b. Co-dominance c. Multiple allele d. Incomplete dominance e. Polygenic inheritance
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Blood group O (genotype ii). Its red blood cells have no A or B antigen, so its cells are not attacked by any recipient's antibodies. O can donate to A, B, AB and O.
Blood group AB (genotype IA IB). Its plasma has no anti-A or anti-B antibodies, so it can receive A, B, AB and O blood safely.
Yes. O plasma contains BOTH anti-A and anti-B antibodies. That is why O can donate to everyone but should receive only from O.
Because AB cells already carry both A and B antigens (co-dominance), the body treats both sugars as 'self' and never makes anti-A or anti-B antibodies against them.
For the donor (O) it is about the ABSENCE of A and B ANTIGENS on the RBCs. For the recipient (AB) it is about the ABSENCE of anti-A and anti-B ANTIBODIES in plasma. NEET often swaps these two.
