For human blood transfusion, which blood group is known as universal donor?

The answer is O- Group. O-negative red cells carry neither the A nor the B antigen, and they lack the Rh (D) antigen. A recipient's anti-A, anti-B or anti-D antibodies therefore have nothing to attack, which is why O-negative blood can be given to almost anyone in an emergency.

Why the others fail:

• B+ Group: its red cells carry B antigen and Rh factor, so it can go only to B+ and AB+ recipients.
• AB Group: its red cells carry both A and B antigens, so donor anti-A and anti-B antibodies would attack them. AB is the universal recipient, not the universal donor.
• A+ Group: its red cells carry A antigen and Rh factor, so it goes only to A+ and AB+ recipients.

Core concept: ABO and Rh antigen-antibody compatibility. Mains link: blood banking and emergency preparedness, and haemolytic disease of the newborn when an Rh-negative mother carries an Rh-positive foetus.

Trap: candidates swap the donor and recipient roles. AB is the universal recipient and O-negative is the universal donor.