The surface of red blood cells have carbohydrates markers called "antigens". Those antigens will produce functional proteins called "antibodies" that will fight foreign invaders. Blood type A has the "A antigen" on its surface and will produce "B antibody". Blood type A cannot receive blood from any type with "B antigen" in its name because "B antibody" will see that donation as foreign type. Blood type B has "B antigen" on its surface and will make "A antibody". A person with blood type B can receive blood from types "Blood type B" and "Blood Type O". Blood type AB is the universal "recipient" because it has no blood "antibody". Blood type O is the universal "donor" because it has no blood "antigen"
