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Eosinophils: Function, Normal Values & Eosinophilia

Eosinophils are white blood cells that play a key role in fighting parasites and mediating allergic reactions. Elevated levels may indicate allergies or other medical conditions.

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Things worth knowing about "Eosinophils"

Eosinophils are white blood cells that play a key role in fighting parasites and mediating allergic reactions. Elevated levels may indicate allergies or other medical conditions.

What Are Eosinophils?

Eosinophils (also called eosinophilic granulocytes) are a type of white blood cell (leukocyte) and belong to the granulocyte family. They are an essential component of the human immune system. Their name comes from their ability to absorb the red dye eosin, which makes them appear characteristic orange-red under a microscope.

In healthy individuals, eosinophils account for approximately 1 to 4 percent of all leukocytes in the blood. They are produced in the bone marrow, circulate briefly in the bloodstream, and then migrate into tissue, where they carry out their primary functions.

Functions of Eosinophils

Eosinophils serve several important roles in the immune system:

Defense against parasites: Eosinophils are particularly effective at combating parasitic worms (helminths) that are too large for individual immune cells to engulf.
Allergic reactions: They are key mediators in allergic inflammatory responses, such as those seen in bronchial asthma, allergic rhinitis, and atopic dermatitis.
Modulation of inflammation: Eosinophils can release both pro-inflammatory and anti-inflammatory signaling molecules, helping to regulate immune responses.
Release of toxic proteins: When needed, they release highly toxic proteins such as Major Basic Protein (MBP) and Eosinophil Cationic Protein (ECP), which can destroy parasites and other pathogens.

Normal Values and Measurement

Eosinophil counts are determined as part of a complete blood count (CBC), specifically the differential blood count. Normal values in adults typically range from 100 to 500 eosinophils per microliter of blood, representing approximately 1 to 4 percent of total leukocytes.

Elevated Eosinophil Levels (Eosinophilia)

An elevated proportion of eosinophils in the blood is referred to as eosinophilia. It is classified as follows:

Mild eosinophilia: 500 to 1,500 cells per microliter
Moderate eosinophilia: 1,500 to 5,000 cells per microliter
Severe eosinophilia (hypereosinophilia): more than 5,000 cells per microliter

Common causes of eosinophilia include:

Allergic diseases (hay fever, asthma, food allergies)
Parasitic infections (e.g., roundworms, hookworms)
Skin conditions such as atopic dermatitis or psoriasis
Chronic inflammatory diseases (e.g., Crohn's disease)
Certain medications (e.g., antibiotics, antiepileptics)
Rarely: hematological conditions such as Hypereosinophilic Syndrome (HES) or leukemia

Low Eosinophil Levels (Eosinopenia)

A deficiency of eosinophils, known as eosinopenia, is less common and can occur during acute bacterial infections, severe physical stress, or as a result of treatment with corticosteroids. In most cases, eosinopenia is of less clinical significance than eosinophilia.

Diagnosis and Clinical Relevance

Elevated eosinophil counts are evaluated by a physician in the context of clinical symptoms and additional laboratory findings. If a parasitic infection or hypereosinophilic syndrome is suspected, further investigations such as stool samples, serological tests, or a bone marrow biopsy may be required.

Treatment

Treatment always targets the underlying cause:

For allergies: antihistamines, corticosteroids, or immunotherapy
For parasitic infections: antiparasitic medications (e.g., albendazole, mebendazole)
For severe hypereosinophilic syndrome: corticosteroids, imatinib, or monoclonal antibodies such as mepolizumab

References

Rothenberg, M.E. - Eosinophilia. New England Journal of Medicine, 338(22):1592-1600, 1998. Available via PubMed.
World Health Organization (WHO) - Blood cell reference ranges and clinical interpretation. Available at: https://www.who.int
Klion, A.D. - Eosinophilia: a pragmatic approach to diagnosis and treatment. Hematology, American Society of Hematology Education Program, 2015:92-97.

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