Erythema Multiforme Major

A 28-year-old woman presents with a 10-day history of painful oral erosions, conjunctivitis, and symmetric target lesions on her dorsal hands and extensor forearms. She reports a history of recurrent herpes labialis 7–10 days prior to each flare. Examination reveals three-zone target lesions (<3 cm), involvement of two mucosae (oral and ocular), and negative Nikolsky sign. The condition marked **B** in the diagram is suspected. Which of the following best describes the pathophysiologic mechanism underlying this condition?