Vitiligo — Symmetric Depigmented Macules

A 27-year-old woman of Indian descent presents to the dermatology clinic with concerns about white patches on her hands that appeared 8 months ago. She reports symmetric bilateral involvement of her dorsal hands, perioral region, and ankles. On examination, the structure marked **A** in the diagram shows milky-white, well-demarcated depigmented macules with sharp margins and no scaling, erythema, or induration. Wood's lamp examination accentuates these lesions. She has a history of Hashimoto's thyroiditis and a family history of type 1 diabetes mellitus. Which of the following best describes the pathophysiology underlying the appearance of the structure marked **A**?