[Eosinophils and eosinophilia]

Eosinophils were previously considered granulocytes involved in host protection against helminth infections and in inflammation related to atopic diseases. Instead, as supported by recent studies, eosinophils are today considered multifunctional cells involved also in homeostasis of the gastrointestinal tract and other organs, conferring innate and adaptive immunity to certain bacteria and viruses, and perhaps in the control of oncogenicity. Unexplained eosinophilia could be an expression of paraneoplastic syndrome or constitutional factors. Irrespective of the underlying conditions and aetiology of eosinophilia, eosinophil-derived substances may induce potentially irreversible organ damage. A diagnostic algorithm is discussed.