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Chris Hatton

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date: 27 February 2021

The histiocytoses are disorders derived from the dendritic cell and monocyte/macrophage lineages, with the classification of this group of disorders relating to the underlying cell of origin. Dendritic cell disorders—there has been much debate about the nature of these conditions, and their status as neoplastic or primary inflammatory diseases; for Langerhans’ cell histiocytosis in particular, there is increasing evidence of their clonal nature, as manifest by recurrent BRAF mutations. Clinical features and diagnosis—these are highly variable and dependent on the sites affected by histiocytic infiltration. Symptoms and signs may include rashes, bony pain, lymphadenopathy, hepatomegaly and splenomegaly, cough and dyspnoea, features of marrow failure, and endocrine presentations (classically diabetes insipidus). Diagnosis typically follows imaging and biopsy, with the demonstration of a histiocytic infiltrate confirmed by immunostaining. Treatment and prognosis—the rarity and heterogeneity of these diseases has made it difficult to achieve a consensus on treatment. For localized disease, curettage, steroid injections, or targeted radiotherapy may be helpful. For more systemic disease, combination chemotherapy is typically used. Treatment schedules differ between adults and children. Prognosis is dependent mainly on the site(s) of involvement. Our expanding appreciation of the molecular basis of these conditions also provides some justification for the use of BRAF inhibitors and other targeted small molecule therapies. Macrophage-related disorders—these include haemophagocytic lymphohistiocytosis, a collection of macrophage-activating syndromes which may be either reactive to underlying inflammatory, infective, or neoplastic disease, or consequent upon a primary genetic lesion affecting cytotoxic T-cell killing function. Rosai–Dorfman disease is a separate macrophage proliferation syndrome, thought to be non-neoplastic, which causes massive cervical lymphadenopathy, usually in children.

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