In this article, the term eosinophilia is defined as an increase in peripheral blood eosinophilic leukocytes to more than 600 cells per microliter (μ L) of blood. Emphasis is placed on the number of eosinophils circulating in the peripheral blood, although an increase in eosinophils can be observed in other body fluids (eg, cerebrospinal fluid [CSF], urine) and many body tissues (eg, skin, lung, heart, liver, intestine, bladder, bone marrow, muscle, nerve).
Eosinophils are derived from hematopoietic stem cells initially committed to the myeloid line and then to the basophil-eosinophil granulocyte lineage. Nonpathologic functions of eosinophils and the cationic enzymes of their granules include mediating parasite defense reactions, allergic response, tissue inflammation, and immune modulation.
Tissues of the pulmonary and gastrointestinal systems are the normal residence for eosinophils, but peripheral, or blood, eosinophilia (absolute eosinophil count [AEC] >600 cells/µL) indicates an eosinophilic disorder. Untreated, the eosinophilia can be categorized as mild (AEC 600-1500 cells/µL), moderate (AEC 1500-5000 cells/µL) or severe (AEC >1500 cells/µL). An increase in tissue eosinophilia may be seen with or without concurrent peripheral eosinophilia.
A secondary or reactive increase in blood eosinophils, tissue eosinophils, or both is associated with a wide variety of infections (especially helminthic parasites), allergic responses, neoplasms, connective tissue disorders, medications and endocrinopathies. Primary eosinophilia is not a reactive phenomenon and can be described as either clonal or idiopathic in nature. If an underlying molecular or cytogenetic abnormality can be identified, the eosinophilia can be designated as a clonal disorder. If reactive causes are ruled out and no underlying clonal origin is proven, the eosinophilia is described as idiopathic.
Given the broad spectrum of conditions linked to eosinophilia, this article emphasizes the diagnostic considerations that clinicians may want to focus on in patients with eosinophilia. The individual disease manifestations and therapies for the dozens of diseases associated with eosinophilia are not described in detail.
