Question

Why is a person having blood group AB called universal recipient and a person with blood group O called a universal donor?

Answer 1

People with blood group O are called universal donors because they can donate blood to all the blood groups. Their red blood cells (RBCs) have no antigens and so cannot be stuck by blood of any other group.
People with blood group AB are called universal recipients because they can receive blood from all 4 blood groups. The red cells in them have no antibodies and therefore it does not stick with blood from other groups.