Bullous Pemphigoid — Tense Subepidermal Bullae

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**?

A 78-year-old woman presents with 6 weeks of intensely pruritic urticarial plaques on her trunk and flexural forearms, evolving into large, tense, fluid-filled bullae 1–3 cm in diameter. The bullae remain intact for days and rupture to leave non-extending erosions. Nikolsky sign is negative. Punch biopsy shows subepidermal blister formation with an eosinophil-rich infiltrate. Direct immunofluorescence of perilesional skin demonstrates a linear band of IgG and C3 along the epidermal basement membrane zone. The structure marked **A** in the diagram represents this clinical and immunological presentation. Which of the following best explains the mechanism of blister formation in this condition?