Eosinophilic Granulocytes – Function and Importance
Eosinophilic granulocytes are white blood cells of the immune system that play a key role in allergic reactions and the defense against parasitic infections.
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Eosinophilic granulocytes are white blood cells of the immune system that play a key role in allergic reactions and the defense against parasitic infections.
What Are Eosinophilic Granulocytes?
Eosinophilic granulocytes, commonly referred to as eosinophils, are specialized white blood cells (leukocytes) that are part of the innate immune system. Their name comes from their ability to absorb the acidic dye eosin during microscopic staining, giving them a characteristic bright pink appearance on blood smears. In healthy adults, eosinophils make up approximately 1–4% of all white blood cells, corresponding to a normal range of 100–500 cells per microliter of blood.
Structure and Development
Eosinophils are produced in the bone marrow from hematopoietic stem cells. Their maturation is primarily regulated by the cytokine interleukin-5 (IL-5). These cells are characterized by a bilobed nucleus and cytoplasm densely packed with large, eosinophilic granules. These granules contain several toxic proteins, including:
- Eosinophil Cationic Protein (ECP)
- Major Basic Protein (MBP)
- Eosinophil Peroxidase (EPO)
- Eosinophil-Derived Neurotoxin (EDN)
These substances are effective at killing parasites, but can also cause tissue damage when released excessively.
Functions in the Immune System
Eosinophilic granulocytes serve several important roles in the human body:
Defense Against Parasites
Eosinophils are particularly effective against multicellular parasites (helminths) that are too large to be eliminated by conventional phagocytosis. They attach to the surface of the parasite and release their toxic granule contents to destroy it.
Involvement in Allergic Reactions
In allergic diseases such as bronchial asthma, allergic rhinitis, or atopic dermatitis, eosinophils are activated by cytokines (IL-4, IL-5, IL-13) and accumulate in affected tissues. There, they contribute to chronic inflammation by releasing pro-inflammatory mediators.
Tissue Modulation and Immune Regulation
Eosinophils are not only effector cells but also play a role in regulating immune responses. They communicate with T cells, mast cells, and dendritic cells and are involved in tissue repair and remodeling.
Eosinophilia: Elevated Eosinophil Count
An elevated number of eosinophils in the blood is called eosinophilia. It is classified as:
- Mild eosinophilia: 500–1,500 cells/µl
- Moderate eosinophilia: 1,500–5,000 cells/µl
- Severe (hypereosinophilic) eosinophilia: more than 5,000 cells/µl
Common causes of eosinophilia include:
- Allergic diseases (asthma, hay fever, food allergies)
- Parasitic infections (e.g., toxocariasis, trichinellosis, schistosomiasis)
- Chronic inflammatory conditions (e.g., Crohn's disease, eosinophilic esophagitis)
- Drug reactions
- Malignancies (e.g., Hodgkin lymphoma, eosinophilic leukemia)
- Hypereosinophilic syndrome (HES)
Eosinopenia: Reduced Eosinophil Count
A deficiency of eosinophils (eosinopenia) is less commonly clinically significant but can occur during severe bacterial infections, sepsis, or as a result of corticosteroid treatment. Corticosteroids suppress the production and release of eosinophils from the bone marrow.
Diagnosis and Clinical Relevance
Eosinophil counts are determined as part of the differential blood count, in which different types of leukocytes are identified and quantified. Additionally, the ECP (eosinophil cationic protein) level in the blood can be measured as a marker of eosinophil activation, which is particularly useful for monitoring asthma. When tissue eosinophilia is suspected (e.g., eosinophilic esophagitis), a biopsy with histological examination is required.
Therapeutic Relevance
Targeted inhibition of eosinophilic inflammatory responses is an important approach in modern medicine. Biologics that specifically target the IL-5 pathway, such as mepolizumab or benralizumab, are successfully used to treat severe eosinophilic asthma and hypereosinophilic syndrome. Corticosteroids remain a key therapeutic option for eosinophilic conditions.
References
- Rothenberg, M. E. (2015). Eosinophilia. New England Journal of Medicine, 372(15), 1459–1461. https://doi.org/10.1056/NEJMra1404688
- Klion, A. D. (2015). Eosinophilia: a pragmatic approach to diagnosis and treatment. Hematology – American Society of Hematology Education Program, 2015(1), 92–97.
- Simon, H. U., Rothenberg, M. E., Bochner, B. S., et al. (2010). Refining the definition of hypereosinophilic syndrome. Journal of Allergy and Clinical Immunology, 126(1), 45–49.
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