Familial eosinophilia is a rare congenital disorder characterized by the presence of sustained elevations in blood eosinophil levels that reach ranges diagnostic of eosinophilia (i.e. 500-1500/microliter) or, far more commonly, hypereosinophilia (i.e. >1,500/microliter). Although high eosinophil levels are associated with certain diseases and thought to contribute to the tissue destruction found in many other eosinophilia-related diseases (see clonal eosinophilia), clinical manifestations and tissue destruction related to the eosinophilia in familial eosinophilia is uncommon: this genetic disease typically has a benign phenotype and course compared to other congenital and acquired eosinophilic diseases.

Individuals with familial eosinophilia exhibit hypereosinophilia presumably from birth (earliest documentation at 4 months of age). Elevated blood levels of eosinophils are remarkably stable over time in affected family members. These individuals are generally detected on routine blood cell counts and at the time of diagnosis present without symptoms or at least no symptoms related to their eosinophilia. Their blood cell counts are typically normal except for eosinophil numbers which range between ~2000 and 5000 cells/microliter (normal <450 cells/microliter). Peripheral blood smears revealed that eosinophils showed normal morphology, a finding distinctly different from many other types of hypereosinophilia in which blood smears commonly shown immature blood cells and eosinophils showing distinctly abnormal areas of cytoplasmic clearing. Ultrastructural changes characteristic of eosinophil activation (e.g. piecemeal degranulation of eosinophils and increases in the number of their lipid bodies), other eosinophil activation markers (e.g. elevated serum levels of eosinophil-derived neurotoxin and major basic protein), and increased expression of a marker for T cell activation, interleukin-2 receptor alpha chain, are absent in individuals with familial eosinophilia but common in other types of hypereosinophilia.

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