Eosinophils are a type of white blood cell that plays a central role in allergic reactions and defence against parasitic infections. They're part of the innate immune system and are produced in the bone marrow before circulating in the blood and accumulating in tissues during an immune response.
Normal range and what elevated eosinophils mean
Normal eosinophil count is 0.1–0.5 × 10⁹/L (or 1–5% of white blood cells). Mild elevation (0.5–1.5 × 10⁹/L) is common and often related to allergies or asthma. Moderate to severe elevation (above 1.5 × 10⁹/L) warrants further investigation as it can indicate parasitic infection, autoimmune conditions or, rarely, haematological malignancy.
Common causes of elevated eosinophils
- Allergic conditions — asthma, allergic rhinitis, eczema, food allergies
- Parasitic infections — especially helminths (worms); less common in urban settings
- Drug reactions — NSAIDs, antibiotics and anticonvulsants are frequent culprits
- Eosinophilic oesophagitis — an increasingly diagnosed inflammatory oesophageal condition
- Connective tissue diseases — Churg-Strauss (eosinophilic granulomatosis with polyangiitis)
- Hypereosinophilic syndrome — rare; can damage the heart, lungs and nervous system
What your doctor will investigate
For mild asymptomatic elevation in a person with known allergies, no further investigation is usually needed. For persistent or unexplained elevation, your doctor may order skin-prick tests, a stool ova and parasites test, lung function tests or — for very high levels — a bone marrow biopsy to exclude haematological disease.