Ten minutes into a transfusion, the patient spikes a fever with back pain and red urine. This is an acute hemolytic reaction — ABO-incompatible blood: recipient antibodies destroy donor red cells intravascularly — fever, back/flank pain, red/brown urine (hemoglobinuria), DIC and shock can follow. Action is instant: STOP the transfusion, keep IV access, notify the blood bank, check the bag and identity — most cases are bedside identification errors. Prevention is the crossmatch ritual — the reaction is almost always human error, not biology.