Final answer:
The first action by the nurse in the event of a transfusion reaction is to stop the transfusion and maintain an IV of normal saline.
Step-by-step explanation:
If a severe transfusion reaction occurs, the first action by the nurse should be to stop the transfusion and maintain an IV of normal saline. Transfusion reactions can be life-threatening, and stopping the transfusion is crucial to prevent further harm. Maintaining an IV of normal saline helps ensure the patient's fluid balance and stability while further interventions are carried out.