Disorders of histiocytic and dendritic cell origin, traditionally referred to as histiocytoses [1], form a group of extremely rare tumours, characterized by variable malignant potential and unpredictable clinical course, from spontaneously regressing to rapidly progressing. While the neoplastic character of some of these entities is still debated, the most commonly acknowledged predictors of malignant behaviour remain as morphological features, i.e. degree of cellular anaplasia. Some of these disorders show predilection for skeletal involvement (Langerhans cell histiocytosis, LCH; Erdheim–Chester disease, ECD) but bone marrow (BM) infiltration is infrequent. In turn, entities classified as frankly malignant (e.g. histiocytic sarcoma, HS) most often form tumoural masses in lymph nodes or at extranodal sites. Interestingly, many of the tumoural lesions in this category demonstrate ability to transdifferentiate to and from other malignancies, including lymphomas and leukaemias. Therefore although primary histiocytic or dendritic cell disorders are most commonly diagnosed in skeletal, soft tissue or lymph node biopsies, BM biopsies (BMB) and aspirates may be obtained for staging purposes or in cases when occurrence of secondary malignancy is suspected.

Infective, granulomatous and benign histiocytic disorders are only rarely an indication for bone marrow (BM) examination and therefore often represent unexpected or incidental findings. Yet since they are frequently linked to a variety of life-threatening underlying conditions, diagnosing such disorders in the BM is almost always significant for the affected patient. This chapter summarizes and illustrates the most common disorders of this type in the BM with a special emphasis on diagnostic and differential diagnostic clues.