Blood contains formed elements like red blood cells, white blood cells, and platelets suspended in plasma. It can be transfused as whole blood or separated into components like packed red blood cells, platelets, and fresh frozen plasma. Proper monitoring during transfusion is important to watch for potential reactions caused by antigen-antibody interactions or other issues like circulatory overload. Immediate reactions require stopping the transfusion and treating the patient, while notifying the physician.