Blood transfusions can save the life of a person, if that person has lost a lot of blood. A person can lose blood from an injury, a trauma or from a medical operation. Blood transfusions can also be applied as a treatment to certain diseases of the blood (like Anaemia, for example).
Blood is not collected and used immediately. Instead it is kept until it is needed. The storage area is informally called a blood bank. By adding anti-coagulant to keep the blood from clotting and refrigerating the blood, it is possible to store it for a few days. Information about transfusion and donation is available at the National Institutes of Health website .
Not all people are able to donate blood to all other people. Testing for blood types usually prevents any bad reaction to a blood transfusion. However, there are special problems for anyone with rare blood groups. It is very important, in order to avoid any complications during a blood transfusion, to detect individuals with rare blood types. The usual tests for ABO blood group system would show them as group O. It may not be possible to give a blood transfusion at all unless the repository has supply of blood which will not cause a reaction.