It depends on how much of the mismatched blood has been transfused. Usually the patient has a severe reaction even with a very small quantity of blood (5-10 millilitres) and the transfusion is stopped. If no one is observing the person receiving blood and larger quantities are transfused, death may result especially if the patient is unconscious or semiconscious and cannot alert anyone. However, we have had patients who have received a full unit of mismatched blood and have survived.