Which blood group is called “universal donor”?
a. A
b. B
c. AB
d. O

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Hint: Universal donor is usually defined as a blood group which can donate blood to anyone and the body doesn’t reject that blood. People with type O blood are called "universal donors" because this type can be used for patients with any blood type. Type O blood is often in short supply in hospitals, due to demand for this universal donor type, according to the Red Cross. Donors with type O- blood have the unique power to help anyone in need of a blood transfusion. Red blood cells from O- donors can be transfused to anyone, regardless of the person's blood type.

Complete answer:

A: - this blood group has A antigen on RBC and an anti B antibody in plasma. So, it can donate only to person with blood group A. So, this option is wrong
B:- this blood group has B antigen on RBC and anti A antibody in plasma. So, it can donate only to people with blood group B. So, this option is also wrong.
AB: - This blood group has antigen A, B and it doesn’t have any antibody that will fight blood groups. So, it can receive blood from any of the blood groups. But can donate blood only to AB blood groups. So, this option is also wrong.
O: - This blood group has no antigen on an anti-A, B antibody so it can donate blood to any group as it has no particular antigen present on it.

Thereby option d is correct that is blood group O as it can donate blood to anyone.

Note: Antigen, substance that is capable of stimulating an immune response, specifically activating lymphocytes, which are the body's infection-fighting white blood cells. Antibodies are Y-shaped proteins produced by B cells of the immune system in response to exposure to antigens. antigens of another blood type will be seen as foreign, and attacked by antibodies.