Page 8 - Eosinofili: Linee Guida

Definitions
Eosinophilia is defined as an elevation of the eosinophil count above levels observed in
healthy subjects, usually taken as above 0.5 × 109/l. Eosinophil counts are higher in
neonates than in adults and the values gradually fall in the elderly. There is no sex or
ethnic variation in the eosinophil count. Definitions of hypereosinophilia (HE) and the
hypereosinophilic syndrome (HES) are based on the proposal by Chusid et al (Chusid et
al, 1975) that required eosinophils to be 1.5 × 109/l or greater in the hypereosinophilic
syndrome with evidence of eosinophil-mediated organ damage or dysfunction after
exclusion of the other potential causes for the organ damage. This criterion was
subsequently accepted in the World Health Organization (WHO) classification of chronic
eosinophilic leukaemia, not otherwise specified (CEL, NOS) (Bain et al, 2008).

Biology
The normal bone marrow contains between 1% and 6% eosinophils and these produce an
eosinophil count in the peripheral blood of 0.05–0.5 × 109/l (Valent et al, 2012).
Eosinophil production in the marrow is tightly controlled by a network of transcription
factors (McNagny & Graf 2002) and is driven by various cytokines, principally
interleukin (IL)-5, IL-3 and granulocyte-macrophage colony-stimulating factor (GM-CSF)
produced by activated T lymphocytes, stromal cells and mast cells, triggering
differentiation and activation (Ackerman & Bochner 2007). Such cytokines are also the
main drivers in reactive eosinophilia in contrast to clonal eosinophilia where tyrosine
kinase fusions are common, typically involving the genes coding for platelet-derived

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