Blood bank. Bags of stored blood in a hospital refrigerator. Blood is collected from donors for transfusion to patients during operations or after accidents where excessive blood loss has occurred. Donated blood is tested for blood-borne diseases and to determine its blood group. The shelves of the blood bank and the bags are labelled with the different blood groups and Rhesus factor types (A, B, AB, O, and positive or negative). It is vital that the same blood group is given to a patient during a transfusion. If a different type is given clotting occurs, which can prove fatal. Blood may be stored whole or may be separated into cells and plasma.