Q. You are called to assess a 51-year-old man who developed a reaction following a blood transfusion. The patient was admitted to the surgical ward one day ago because of a traffic accident that resulted in a splenic laceration, mandating urgent splenectomy. Postoperative routine lab analysis reveals critical anemia, so you order a blood transfusion. One hour after the transfusion, the nurse notices an elevation in the patient's temperature. Upon examination, the patient appears flushed; however, he does not display jaundice or any skin lesions. Which of the following best explains the above patient complication?