Eosinophils are differentiated white blood cells containing cytoplasmic granules that stain pink with eosin. Differentiation and survival are promoted by cytokines, especially interleukin-5; therefore, an increase in eosinophils reflects inflammation. Eosinophil granules contain proteins that are cytotoxic to parasites and have a role in maintaining the body’s inflammatory response, but they can also cause organ damage when released in tissues. Recognition of eosinophilia is important for identifying cause and mitigating ongoing tissue damage.

Eosinophils dwell primarily in tissues throughout the body and compose only a small percentage of peripheral blood leukocytes. The absolute eosinophil count (AEC) is normally less than 450/μL (<0.45 × 109/L). Eosinophilia is most often defined as an AEC greater than 500/μL (>0.5 × 109/L) and can be divided into 3 categories: mild (500–1,500/μL [0.5–1.5 × 109/L]), moderate (1,500–5,000/μL [1.5–5.0 × 109/L]), and severe (>5,000/μL [>5.0 × 109...

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