I recently had major surgery and got two units of blood in during the operation and two more post-op. Post-op before I got the blood, they typed my blood again, and a nurse stayed in the room while I got the blood and I wondered why. This comment makes it clear.

Close observation for 15 minutes is typical for any blood transfusion. You do a set of pre-transfusion vitals, vitals when the blood hits the vein, vitals every 5 minutes until 15 minutes is up, vitals every 15 minutes until the blood is done. Ask any nurse why they hate running blood.

Depending on the severity of the reaction, blood will either be stopped or the patient will be loaded up with Benadryl and Tylenol with the blood running at a slower rate.