Why is a person with type O blood called universal donor and AB receiver?

A person with type O blood is called a universal donor because their red blood cells can be safely transfused to people with any other blood type. This is because type O red blood cells do not have any of the A, B, or Rh antigens that can cause an immune reaction in people with other blood types.

On the other hand, a person with type AB blood is called a universal recipient because they can safely receive blood transfusions from people with any other blood type. This is because type AB plasma does not have any of the anti-A or anti-B antibodies that can attack red blood cells of other blood types.