A 72-year-old man presents with a 6-week history of severe pruritus followed by the appearance of fluid-filled blisters on his abdomen, inner thighs, and axillae. On examination, the lesions marked **A** in the diagram are dome-shaped, intact, and do not rupture easily when pressure is applied. His Nikolsky sign is negative. He has been on vildagliptin for type 2 diabetes for the past 8 months. Direct immunofluorescence of perilesional skin shows linear IgG and C3 deposition along the basement membrane zone. Which of the following best explains the pathogenesis of the lesions marked **A**?