The histiocytoses are a group of rare disorders that involve an abnormal accumulation of a specific cell type (the histiocyte) in various organs. Histiocytes are mainly derived from the bone marrow, developing from a common hematopoetic progenitor into two distinct types, the monocyte/macrophage system and the dendritic cells (DC), of which Langerhans cells (LCs) are one cell type. Despite advances in the study of the histiocytoses, their etiology remains elusive, their pathogenesis is poorly understood, their classification is constantly changing and their treatment is far from standardized. Of the histiocytoses, the DC group affects the skin more frequently than the hemophagocytic or malignant variants, particularly in children. The two most frequent dendritic histiocytoses are an accumulation of pathologic LCs, that manifests as LC histiocytosis (LCH), and a proliferation of dermal/interstitial dendrocytes, that presents in the skin and other organs as xanthogranulomas. Histiocytosis affects the skin more commonly than any other organ except bone. Due to the ease of skin biopsy, it is the most useful way of establishing the diagnosis of LCH. LCH of the skin has a wide range of presentations, but neither the type of lesion nor the histology is helpful in evaluating the prognosis.

This chapter will discuss the pathology, clinical appearance, appropriate investigations, treatment and prognosis of skin histiocytosis in children and adults.

LCH in infancy and childhood

Epidemiology

LCH is a rare disease affecting 2–7 children per million per year (Carstensen and Ornvold, 1993; Nicholson et al., 1998) or 1 in 25,000 children per year (Favara et al., 1997).