Blood transfusion reactions may occurTrusted Source while you’re receiving blood or immediately after. They may also occur hours, days, or weeks later. This is called a delayed transfusion reaction. Transfusion reactions may range in severity from minor to life threatening. A doctor or nurse will stay with you while you receive the transfusion. They will check your vital signs and watch for symptoms that you may be having a reaction to. Transfusion reaction symptoms may include: back pain dark urine chills fainting or dizziness fever flank pain skin flushing shortness of breath itching