Before a blood transfusion, a technician tests your blood to find out what blood type you have (that is, A, B, AB, or O and Rh-positive or Rh-negative). He or she pricks your finger with a needle to get a few drops of blood or draws blood from one of your veins.
The blood type used in your transfusion must work with your blood type. If it doesn't, antibodies (proteins) in your blood attack the new blood and make you sick.
Some people have allergic reactions even when the blood given does work with their own blood type. To prevent this, your doctor may prescribe a medicine to stop allergic reactions. (For more information, see "What Are the Risks of a Blood Transfusion?")
If you have allergies or have had an allergic reaction during a past transfusion, your doctor will make every effort to make sure you're safe.
Most people don't need to change their diets or activities before or after a blood transfusion. Your doctor will let you know whether you need to make any lifestyle changes prior to the procedure.