Definition

The term “histiocyte” (tissue cell) has evolved and is now often used as a collective noun for two related groups of immune regulatory cells, the monocyte/macrophage and the dendritic cell–accessory antigen presenting cells [1, 2]. The histiocytic proliferations of childhood, therefore, encompass the benign and malignant accumulations of monocyte-macrophages and of dendritic cells, though distinguishing the borderline between their reactive and neoplastic states still seems to be elusive [3, 4].

Origins and variety of histiocytes

Macrophages and dendritic histiocytes appear to share origin from a common marrow precursor that gives rise to divergent lines of differentiation [5], although a dendritic cell precursor independent of the myeloid and lymphoid line has been identified [6]. A competing theory is that various cell lines, lymphoid and myeloid, can produce cells that have macrophage and/or dendritic cell features as an adaptive state rather than as separate lineages [7–9]. The macrophages are most important in immediate innate immunity, with rapid response that leads to particulate removal and destruction, whereas dendritic cells preserve the antigenic information for use in adaptive responses [10]. Macrophage-derived pro- and anti-inflammatory cytokines mediate an ongoing process that interacts with the dendritic, antigen-presenting cells and also effects reconstitution of damaged tissues [11]. The distinctions between the monocyte-macrophages and dendritic cells are probably not as rigid as supposed, with a capability for some cross-differentiation until late in the process, and back-and-forth modulation [12, 13].