Which blood type is known as the universal donor?

The designation of the universal donor blood type refers to the ability of that type to be transfused into individuals with any other blood type without triggering an immune response. Blood type O is labeled as the universal donor because it lacks A and B antigens on the surface of its red blood cells. This means that individuals with blood type O do not possess the markers that could provoke an immune reaction in recipients with blood types A, B, or AB.

When a person receives a blood transfusion, their immune system can react to foreign antigens, which can lead to serious complications. Since type O blood has no antigens that might clash with A or B antigens found in other blood types, it is safe to transfuse into anyone, making type O compatible with all other blood types.

In contrast, blood types A, B, and AB each have specific antigens that can provoke an immune response if given to someone with a different blood type. For example, type A has A antigens and can cause an immune reaction in type B or type AB recipients, who have B antibodies. Therefore, while those blood types can receive from specific types, they cannot be considered universal donors.