A 48-year-old woman of South Asian descent presents with a 3-month history of painful oral erosions initially thought to be recurrent aphthous ulcers. Over the past 4 weeks, flaccid bullae have appeared on her trunk and flexural areas, rupturing easily with minimal trauma. On examination, gentle lateral pressure on apparently normal-appearing skin of the forearm results in immediate epidermal separation and blister formation, as marked **D** in the diagram. Which of the following best explains the pathophysiologic basis of this clinical finding?