Eosinophil – Definition, Functions, Features, Structure

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What are Eosinophils?

  • Eosinophils, also known as leukocytes, are a type of white blood cell found in our bodies.
  • Eosinophils, also known as eosinophiles or, less frequently, acidophils, are a type of white blood cell (WBC) and a component of the immune system responsible for battling multicellular parasites and certain illnesses in vertebrates.
  • In addition to mast cells and basophils, they regulate allergy and asthma-related pathways.
  • They are granulocytes that form in the bone marrow during hematopoiesis before migrating into the blood, where they undergo terminal differentiation and do not proliferate. They make up between 2% and 3% of WBCs.
  • These cells are eosinophilic or “acid-loving” because of their enormous acidophilic cytoplasmic granules, which demonstrate their love for acids through their affinity for coal tar dyes: This affinity is what causes them to appear brick-red after being stained with eosin, a red dye, using the Romanowsky process.
  • The staining is concentrated in tiny granules inside the cytoplasm of the cell, which contain numerous chemical mediators, including as eosinophil peroxidase, ribonuclease (RNase), deoxyribonucleases (DNase), lipase, plasminogen, and major basic protein.
  • Following eosinophil activation, degranulation releases these toxic mediators, which are harmful to both parasite and host tissues.
  • Eosinophils belong to the granulocytic cell category, which also includes neutrophils and basophils.
  • Eosinophils have a bilobed nucleus and granulated cytoplasm, constituting a bilobed shape.
  • The granular structure of the cytoplasm is a characteristic shared by all cells under this category.
  • The granulated cytoplasm is stained with eosin, which imparts a red hue to the cytoplasm. The method of staining utilised is the Romanowski procedure. This is the origin of the term “eosinophil.”
  • Eosinophils can be defined as the motile phagocytic cell that is involved in an immune response. Despite being phagocytic in nature, they are not the body’s predominant phagocytic cells; rather, they serve a crucial part in the body’s defence against parasite infection.
  • Eosinophil granules contain a lytic enzyme that can harm the membrane of a pathogen. They also enhance the host’s resistance to nematodes.
  • Eosinophils are also implicated in the inflammatory response; excessive eosinophils harm the host’s immune system.
  • In healthy individuals, eosinophils comprise 1–3% of white blood cells and range in size from 12 to 17 micrometres with bilobed nuclei.
  • While neutrophils are discharged into the bloodstream, eosinophils reside in the tissue. In normal circumstances, they are located in the thymus medulla and the junction of the cortex and medulla, as well as in the lower gastrointestinal tract, ovaries, uterus, spleen, and lymph nodes, but not in the lungs, skin, oesophagus, or other internal organs[vague].
  • Disease is linked to the prevalence of eosinophils in these latter organs. Patients with eosinophilic asthma, for example, have elevated amounts of eosinophils, which cause inflammation and tissue damage, making it more difficult for them to breathe.
  • In the absence of stimulation, eosinophils can stay in the circulation for 8–12 hours and in tissue for a further 8–12 days.
  • Ex-vivo culture tests revealed that eosinophils are distinct granulocytes with the ability to persist for extended periods of time after maturation, as revealed by groundbreaking research in the 1980s.
3D rendering of eosinophil
3D rendering of eosinophil | Image Source: Blausen Medical, CC BY-SA 4.0 https://creativecommons.org/licenses/by-sa/4.0, via Wikimedia Commons

Characteristics of Eosinophil

The eosinophil possesses the following characteristics:

  • These cells are granulocytic phagocytes.
  • They have acidic cytoplasm, and eosine is the staining agent.
  • They measure 8 micrometres in diameter.
  • They make from 1% to 5% of total leukocytes.
  • They play a crucial role in the host’s immune response against endoparasitic infections as schistosomiasis and helminthic.
  • They are derived from myeloid progenitor-lineage Hematopoietic Stem Cells (HSC).
  • They possess IgG and IgE membrane-bound receptors.
  • In the bone marrow, SCF, IL-3, IL-4, GM-CSF, and CCL11 regulate the differentiation and development of eosinophils from myeloid progenitors.
  • IL-5 and CCL11 coordinate the ultimate differentiation, maturation, and bloodstream release of eosinophils.
  • During the immunological reaction, the granular contents are liberated.
Eosinophil | Image Source: Bobjgalindo, CC BY-SA 4.0 https://creativecommons.org/licenses/by-sa/4.0, via Wikimedia Commons

Structure of Eosinophils

  • Eosinophils are granulocytes with a diameter of between 10 and 16 m. The cells have segmented or bilobed nuclei, and the ratio of nuclei to cytoplasm is approximately 30%.
  • Important to the morphology of eosinophils is the presence of particular cationic proteins in their cytoplasmic granules.
  • The granules of eosinophils are the most distinguishing characteristic of the cells. These granules possess a unique crystalloid core, a thick matrix surrounded by a membrane. The core of eosinophils is responsible for their cardinal features.
  • In addition to proteins, cytokines, chemokines, and enzymes, the granules include many mediators that can produce inflammation and tissue damage.
  • Golgi bodies, endoplasmic reticulum, and mitochondria manufacture proteins and ATPs in the cell and are located in the cytoplasm.
  • The histological structure of eosinophils is dependent on the level of activation of the cells, as the number of vesicles in the cells can only be observed when the cell is degranulating piecemeal.
  • Eosinophil plasma membranes include multiple receptors that are specialised for certain cytokines that govern cell maturation and other physiological processes.

Granular Content of Eosinophil

The granule in the cytoplasm of the eosinophil is the primary enzyme responsible for the eosinophil’s function. The following are contained in the granule:

  • Mazor Basic Protein – Also known as MBP, is a protein that stimulates the degranulation of mast cells and basophils. These proteins are also involved in the remodelling of peripheral nerves.
  • Eosinophil Cationic Protein – These proteins (ECP) are accountable for producing pores in the pathogen’s membrane. The creation of pores is crucial because it allows the cytotoxic molecule of the host to enter the pathogen’s body and induce cell death via membrane disintegration. It stimulates fibroblast glycosaminoglycan secretion. Additionally, it inhibits antibody production and T-cell proliferation.
  • Eosinophil Peroxidase- It encourages the development of reactive oxygen species and reactive nitrogen species intermediates. The synthesis of these substances in the pathogen’s body induces oxidative stress, leading to apoptosis and necrosis-mediated cell death.
  • Eosinophil-Derived Neurotoxin – The Eosinophil-Derived Neurotoxin is an antiviral RNAse. They typically inhibit the pathogen’s transcription machinery.

Functions of Eosinophil

  • Among the functions of the eosinophil cell is its ability to eliminate fibrin during inflammation.
  • They contribute to neoplasia
  • They are published pubertal development of the mammary gland
  • Increased absolute eosinophil count is the most important cause of asthma.
  • For the diagnosis of disorders such as Cushing’s syndrome, eczema, and leukaemia, the absolute eosinophil count is employed.
  • They contribute to the inflammatory response
  • In addition, they serve as antigen-presenting cells to T-cells.
  • They participate in the rejection of allografts during raft transplantation.
  • The effector function of these cells includes the synthesis of the following compounds:
    • The following cytokines are produced by these cells: IL-1, IL-2, IL-4, IL-5, IL-6, IL-8, and IL-13.
    • They are involved in TNF alpha production (Tumor Necrosis Factor).
    • They generate development factors.
    • They generate elastase.
    • They are also involved in the generation of lipid mediators such as eicosanoids and prostaglandin from leukotriene (e.g., LTC4, LTD4, LTE4).

What is Eosinophilia?

  • It is a disorder characterised by an elevated absolute eosinophil count in the blood.
  • Absolute eosinophil count is a blood test that measures the cell concentration.
  • When a person has more than 500 eosinophils per microliter, eosinophilia is identified.
  • Increased eosinophil count may be an indication of Acute hypereosinophilic syndrome, eczema (skin disorder, itchy or irritated skin), asthma, or hay fever, as well as many autoimmune disorders.
  • In addition to being a sign of leukaemia, eosinophilia is also used to diagnose the early stages of Cushing’s illness, which is characterised by an abnormally elevated cortisol concentration.
  • The reduced number of eosinophils or their low concentration can be related to alcohol consumption. A individual suffering from Cushing’s syndrome also has abnormally low eosinophil levels.
  • In general, low levels of eosinophils are not life-threatening unless all other white blood cell counts are also abnormally low. Typically, these disorders result from an anomaly or dysfunction in the bone marrow.
  • Eosinophilic disorders in the body include eosinophilic esophagitis, a disorder of the gastrointestinal tract, specifically the oesophagus; this is commonly known as esophageal eosinophilia.
  • Eosinophilic colitis, which is eosinophilia of the large intestine, is another disorder. Eosinophilia can affect a variety of organs, however these are the most commonly affected.

Types of Eosinophilia

Eosinophilia can be primary or secondary:

Primary causes

  • Leukemia eosinophilic chronique.
  • Myeloid and lymphoid tumours having PDGFRA, PDGFRAB, or FGFR1 gene rearrangements.
  • Eosinophilia hereditary
  • Syndrome of idiopathic hypereosinophilia.

Secondary causes

  • Parasitic infestations: Infestations caused by parasites include ancylostomatidis, ascariasis, cysticercosis, echinococcosis (hydatid cyst), schistosomiasis, strongyloidiasis, trichinellosis, and visceral larva migrans (toxocariasis).
  • Fungal and bacterial infections: Infections caused by fungi and bacteria include bronchopulmonary aspergillosis, chronic tuberculosis (rarely), coccidioidomycosis, disseminated histoplasmosis, and scarlet fever.
  • Allergic disorders: Bronchial asthma, hay fever, Stevens-Johnson syndrome, medication and food allergic reactions, DRESS syndrome are examples of allergic illnesses.
  • Skin diseases: Atopic dermatitis, eczema, pemphigus, Mycosis fungoides, and Sezary syndrome are skin illnesses.
  • Host-versus-graft response
  • Connective tissue disease: Diseases of the connective tissue include Chrug-Strauss syndrome and eosinophilic myalgia syndrome.
  • Miscellaneous: pulmonary eosinophilia reactive, tropical eosinophilia, pancreatitis, eosinophilic gastroenteritis.

Reason for high Eosinophils counts

Eosinophils are a type of white blood cell that play a role in the body’s immune response to allergens and parasites. A high count of eosinophils, a condition called eosinophilia, may indicate an underlying health problem. Here are some possible reasons for a high eosinophil count:

  1. Allergic reactions: Allergies to substances such as pollen, food, or medications can trigger the release of eosinophils and lead to a high eosinophil count.
  2. Asthma: Asthma is a chronic respiratory condition that involves inflammation of the airways. Eosinophils are often involved in this inflammatory response, leading to a high eosinophil count.
  3. Parasitic infections: Certain parasitic infections, such as roundworms or hookworms, can trigger a high eosinophil count.
  4. Autoimmune disorders: Some autoimmune disorders, such as eosinophilic granulomatosis with polyangiitis (EGPA), can cause a high eosinophil count.
  5. Skin conditions: Certain skin conditions, such as eczema, can cause a high eosinophil count.
  6. Certain medications: Certain medications, such as antibiotics and nonsteroidal anti-inflammatory drugs (NSAIDs), can cause a high eosinophil count as a side effect.
  7. Blood disorders: Certain blood disorders, such as lymphoma or leukemia, can cause a high eosinophil count.

Reason for low Eosinophils counts

Eosinophils are a type of white blood cells that play a role in the body’s immune response to infections and inflammation. A low count of eosinophils, a condition called eosinopenia, may indicate an underlying health problem. However, eosinopenia itself is not a disease, but a symptom of an underlying condition.

Some of the conditions that can lead to low eosinophil count include:

  1. Acute bacterial infections: Such as sepsis, bacterial pneumonia, and meningitis, can cause a temporary decrease in eosinophil count.
  2. Certain medications: Such as corticosteroids, which are commonly used to treat inflammation and allergies, can suppress eosinophil production and lead to a low count.
  3. Stress: Prolonged stress can decrease eosinophil count.
  4. Certain cancers: Some cancers, such as leukemia and lymphoma, can lead to a low eosinophil count.
  5. Overwhelming infections: Viral infections such as HIV, measles, mumps, and rubella, can cause a temporary decrease in eosinophil count.
  6. Autoimmune disorders: Such as lupus and rheumatoid arthritis, can cause a low eosinophil count.
  7. Parasitic infections: Such as hookworms, roundworms, and tapeworms, can cause a decrease in eosinophil count.
  8. Congenital disorders: Rare congenital disorders, such as Job’s syndrome, can lead to a low eosinophil count.


What are eosinophils?

Eosinophils are a type of white blood cell that play a role in the body’s immune response to allergens and parasites.

What is the normal range for eosinophil count?

The normal range for eosinophil count is usually between 50 and 500 eosinophils per microliter of blood.

What does a high eosinophil count indicate?

A high eosinophil count may indicate an underlying health problem, such as an allergic reaction, asthma, or parasitic infection.

What does a low eosinophil count indicate?

A low eosinophil count may indicate an underlying health problem, such as a bacterial infection, stress, or certain medications.

How is an eosinophil count measured?

An eosinophil count is measured using a blood test called a complete blood count (CBC).

Can diet affect eosinophil counts?

There is limited evidence to suggest that diet may affect eosinophil counts, particularly in individuals with allergies or asthma.

What are the symptoms of eosinophil-related conditions?

Symptoms of eosinophil-related conditions can vary depending on the underlying cause, but may include respiratory symptoms, skin rashes, gastrointestinal issues, and fatigue.

How are eosinophil-related conditions treated?

Treatment for eosinophil-related conditions depends on the underlying cause, but may include medications such as corticosteroids, antihistamines, and anti-parasitic drugs.

Can stress affect eosinophil counts?

Prolonged stress may decrease eosinophil counts, but the link between stress and eosinophils is not fully understood.

Are eosinophils always harmful?

While eosinophils can contribute to inflammation and tissue damage in certain conditions, they also play a vital role in the body’s immune response to parasitic infections.


  • Kovalszki A, Weller PF. Eosinophilia. Prim Care. 2016 Dec;43(4):607-617. doi: 10.1016/j.pop.2016.07.010. Epub 2016 Oct 14. PMID: 27866580; PMCID: PMC5293177.
  • Kanuru S, Sapra A. Eosinophilia. [Updated 2022 Sep 26]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK560929/
  • https://www.news-medical.net/life-sciences/Eosinophil-Function.aspx
  • https://www.vedantu.com/biology/eosinophils
  • https://microbenotes.com/eosinophils/
  • https://www.mayoclinic.org/symptoms/eosinophilia/basics/causes/sym-20050752
  • https://www.cincinnatichildrens.org/service/c/eosinophilic-disorders/conditions/eosinophil
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