MODEL RELEASED. Blood bank. Technician removing a bag of donated blood from the cold store of a blood bank. Prior to storage donated blood is separated into its components (including red blood cells, seen here) and tested to determine its blood group and for the presence of any pathogens. Red blood cells can be kept in a cold store for up to 42 days. Blood transfusions are used to replace blood lost during surgery or because of trauma. It is important that the blood group of the donor and recipient's blood are compatible, as incompatible transfusions can cause fatal immune reactions.