Biology · Body Fluids and Circulation · NEET
It is about ANTIGENS on the donor RBC. Group O RBCs have neither A nor B antigen on their surface. When O blood is given to any recipient, there is no antigen for the recipient's anti-A or anti-B antibody to grab, so no clumping happens. AB RBCs carry both A and B antigens, so almost every recipient's plasma would attack them — that is why AB cannot be a universal donor.
It is about ANTIBODIES in the recipient plasma. AB plasma has NO anti-A and NO anti-B antibody (nil). So whatever donor RBCs come in, there is nothing in AB plasma to clump them, and AB can take blood from A, B, AB or O. Group O plasma has BOTH anti-A and anti-B, so O plasma would attack A, B and AB donor cells — O cannot be a universal recipient.
Strictly, O NEGATIVE is the true universal donor because it also lacks the Rh (D) antigen, so it will not trigger any Rh reaction either. NCERT teaches 'O' as the universal donor at the ABO level, but for real transfusion the safest is O-negative. NEET usually accepts 'O group' as the universal donor unless the question specifically brings in the Rh factor.
No — this is the most common trap. Group O RBCs have no antigen, but group O PLASMA still contains both anti-A and anti-B antibodies. 'Universal donor' only depends on the donor's RED CELLS (antigens), because in a small transfusion the donor's plasma is diluted. So O can donate to all, but O cannot receive from all.
For NEET's basic logic, only the donor's ANTIGENS (on RBCs) and the recipient's ANTIBODIES (in plasma) are matched. The donor's plasma antibodies get heavily diluted in the recipient's large blood volume, so they are ignored in the textbook rule. That is exactly why O (no antigen) donates to all and AB (no antibody) receives from all.
Persons with "AB" blood group are called as "Universal recipients". This is due to:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Blood group O. Its RBCs have neither A nor B antigen, so no recipient's plasma antibody can attack the donated cells. O (specifically O-negative) can be given to any group.
Blood group AB. Its plasma has no anti-A or anti-B antibody, so it can receive blood from A, B, AB and O groups.
Donation depends on the donor RBC antigens (O has none, so safe to give). Receiving depends on the recipient's plasma antibodies (O plasma has both anti-A and anti-B, which would attack A, B and AB cells), so O cannot receive from all.
AB has both A and B antigens on its RBCs and NO antibodies in its plasma (neither anti-A nor anti-B).
O-negative is the true universal donor because it also lacks the Rh antigen. NCERT lists 'O' at the ABO level; O-negative is safest for actual transfusion.
