Hand-Schuller-Christian Disease

A 5-year-old boy presents with a 3-month history of polyuria, polydipsia, and bilateral proptosis. His parents report he was hit in the head during play 6 months ago but had no serious injury. Lateral skull radiograph shows multiple well-circumscribed lytic lesions with characteristic beveled edges (uneven destruction of inner and outer tables creating a 'hole-within-a-hole' appearance) in the calvarium, and a destructive lesion in the temporal bone. Urine specific gravity is 1.002 and fails to concentrate with water deprivation but normalizes with desmopressin. Biopsy of a skull lesion reveals sheets of cells with grooved nuclei, abundant eosinophils, and CD1a+ immunostaining. The pathologic process marked **A** in the diagram is a clonal neoplastic proliferation of which cell type?