Histiocytic infiltration usually represents a reactive process, the reasons for which are not well defined in rodents. Compounds that are cytotoxic and cause necrosis of the bone marrow may result in increased numbers of macrophages, which serve to remove the cellular debris. In addition, any condition capable of causing bone marrow injury or inflammation can result in increased macrophages. In other species (humans and dogs), noninflammatory histiocytic infiltrates have been associated with bone marrow hyperplasia, ineffective hematopoiesis, or increased breakdown of blood cells.
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