A 28-year-old woman presents to the emergency department with acute onset of angioedema involving the lips, tongue, and throat 15 minutes after receiving intravenous penicillin for a urinary tract infection. She reports a prior episode of similar swelling after consuming shellfish 2 years ago. On examination, she is anxious, with audible stridor and mild wheezing. Blood pressure is 95/60 mmHg, heart rate 118/min. Within minutes of receiving intramuscular epinephrine, her symptoms begin to resolve. What is the immunological mechanism underlying this acute reaction?