Eosinophils (absolute and %) typically make up ~1–3% of white blood cells, with an absolute range up to ~0.5 × 10⁹/L (500 cells/μL). They are involved in allergic responses and defense against parasites.

Mild elevations are most commonly linked to allergies or atopic conditions. More significant or persistent elevations may suggest parasitic infection, medication reactions, or, less commonly, systemic or hematologic disease. Low levels are common with stress or steroid exposure and are usually not clinically relevant.