Blood transfusions can save the life of a person, if that person has suffered from a severe blood loss. This blood loss can occur due to an injury, a trauma or if that person had a medical operation performed. 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. Blood types prevent that.