Eosinophil, basophil, and mast cell infiltrates in the bone marrow: crossing the boundaries of diagnosis

An increase in eosinophils, basophils, and/or mast cells in the bone marrow may pose considerable diagnostic problems for the hematopathologist. In a first step, it is crucial to classify these changes as either being reactive or neoplastic. A slight increase in eosinophils is often seen in reactive states of myeloid hyperplasia while an increase in basophils is usually indicative of a neoplastic hematologic disorder. Regarding mast cells, the presence of compact infiltrates is the major criterion for systemic mastocytosis whereas a diffuse interstitial increase is often encountered in reactive states of mast cell hyperplasia. When a neoplastic disorder is diagnosed, definitive classification depends on immunohistochemical, molecular, and cytogenetic markers. The use of a limited panel of antibodies against basophil-, eosinophil-, and mast cell-related antigens like 2D7, eosinophil major basic protein, tryptase, and CD117 (KIT) is sufficient to identify diffuse or compact infiltrates and to provide semiquantitative analysis of accumulations of neoplastic or reactive cells. We therefore recommend the use of such markers which may increase the value of hematopathologic investigations and thereby are of major help to improve diagnosis and prognostication in these patients.