When people donate blood, it is stored in a blood bank and used for blood transfusions. A blood transfusion boosts blood levels that are low, either because a body isn't making enough or a patient has lost blood owing to surgery, injury or disease. In some cases, typically those involving elective surgery, a blood transfusion is done with blood the patient has donated for themselves ahead of time.

During a typical blood transfusion, the needed parts of blood are delivered through an intravenous (IV) line that's placed in one of the arm veins. A blood transfusion usually takes one to four hours, though in an emergency it can be done much faster.

Why you may need a blood transfusion

People receive blood transfusions for many reasons. Blood transfusions provide the part or parts of blood that are most needed in a patient: red cells, white cells, plasma or platelets. Issues that most commonly require blood transfusions are:

Surgery
Injury
Anemia
Gastrointestinal bleeding
Cancer
Illness

Blood components each have important functions

Red blood cells transport oxygen from the lungs to the tissues; too few red cells lead to anemia, tiredness, malaise and pale skin.
Platelets help stop bleeding by plugging holes in the blood vessel wall; patients with too few platelets bleed more easily and develop "petechiae," which are flea-bite size bleedings in the skin and may show up as small purple dots on the skin.
White blood cells fight infections by eating and killing bacteria.
Plasma carries all three of the above components. The primary purpose of plasma is to transport nutrients, hormones and proteins to the parts of the body that need it.