A 12-year-old child presents with annular, skin-colored papules over the dorsal surfaces of both hands that have persisted for 3 months. Dermoscopic examination and biopsy are performed. The histological section shows a characteristic ring-like arrangement of inflammatory cells surrounding a central zone of degenerated collagen and mucin. The structure marked **A** in the diagram represents the inflammatory cells forming this distinctive palisade pattern. Which of the following best describes the cell type and its role in the pathogenesis of this lesion?