Eosinophils
White blood cells involved in allergies, parasite defense and inflammation
Also known as
Definition
Eosinophils are one of the five types of white blood cells (leukocytes). They defend against parasites and certain infections, take part in allergic reactions and help control inflammation. The blood differential reports them as a percentage of all white blood cells; multiplying that share by the white blood cell count gives the absolute eosinophil count. Under the microscope, their granules stain orange-red.
| Parameter | Value |
|---|---|
| Unit | cells/µl or ×10³/µl (absolute); % of white blood cells (relative) |
| Reference Range | Typical for adults: absolute below 500/µl (below 0.5 ×10³/µl), relative 1–4 % of white blood cells. Labs differ slightly; the range printed on your own report is what counts. Discuss any deviation with your doctor. |
Agree on your personal target range with your doctor.
500/µl equals 0.5 × 10⁹/l or 0.5/nl. US reports use cells/mcL.
↓ What a low value means
A lower-than-usual count can be linked to alcohol intoxication, increased production of certain steroid hormones in the body such as cortisol, or taking steroid medicines. In addition, a strong rise in another type of white blood cell can lower the percentage of eosinophils without changing their absolute number. Your doctor puts the result in context.
↑ What a high value means
A raised count is called eosinophilia. It can be linked to allergic conditions such as hay fever, asthma or eczema, and also to parasite infections (such as worms), fungal infections, autoimmune diseases, adrenal insufficiency or inflammation of blood vessels, and less often to leukemia, lymphoma or hypereosinophilic syndrome. Some medicines, including certain antibiotics, interferon or psyllium-containing laxatives, can also raise the value. The count alone does not show whether an allergy or a parasite is behind it; discuss any deviation with your doctor.
When to test
Eosinophils are counted as part of the blood differential, often at routine check-ups. The absolute count is measured specifically when the differential was abnormal or when an allergy, parasite infection or another relevant condition is suspected. Usually no special preparation is needed; tell the practice about all medicines you take, including over-the-counter ones.
Frequently asked questions
Does a raised eosinophil count mean I have an allergy? +
Not on its own. Eosinophils often rise with allergies, but also with parasite infections, autoimmune diseases, some medicines and rare blood disorders. The value is therefore only one piece of the picture; your doctor clarifies the cause with further findings.
Why does my report show a percentage and an absolute number? +
The percentage is the share of eosinophils among all white blood cells. The absolute number results from multiplying that share by the white blood cell count. Because percentages shift when another cell type rises sharply, the absolute number is often more informative.
Do I need to prepare for the test? +
Usually not; the blood differential requires no special preparation. Do mention which medicines you take, including over-the-counter ones, because some can change the value.
How do eosinophils differ from basophils? +
Both are white blood cells and both take part in allergic reactions. Eosinophils also defend against parasites and help control inflammation; basophils release enzymes during allergic reactions and asthma attacks. The blood differential reports them separately.
Eosinophils
Sources
Last updated: October 05, 2026
This information is for orientation only and does not replace medical advice. Reference ranges can vary by laboratory, method and country.