What Are Eosinophils? Definition, Features & Their Role in Immunity
Eosinophils are the type of white blood cells also known as leukocytes found in our bodies. The eosinophils are categorized under the group of granulocytic cells, these groups of cells include neutrophils, basophil, and eosinophil. Eosinophil is a bilobed structure, that is they have a bilobed nucleus, have a granulated cytoplasm. The granular structure of the cytoplasm is the common feature of all the cells of this category. The granulated cytoplasm gets stained by eosin, giving red color stain to the cytoplasm. The staining method used is the Romanowski method. This is the reason behind the name “eosinophil”.
Eosinophils meaning can be described as the motile phagocytic cell which is involved in an immune response, despite being phagocytic in nature they are not major phagocytic cells of the body rather they play a vital role in defense against parasitic infection. The granule of the eosinophil contains a lytic enzyme that can damage the pathogen’s membrane. They also improve host defence against nematodes. Eosinophils are also involved in the inflammatory response, increased eosinophils cause damage to the host immune system.
Features of Eosinophil
There are the following feature of an eosinophil, they are listed as follows:
They are granulocytic phagocytic cell
They have acidic cytoplasm, eosine is the dye used for staining.
They have a diameter of 8 micrometres.
They constitute 1 to 5% of total leukocytes
They play a major role in the immune response of the host against endoparasitic infection such as schistosomiasis and helminthic.
They are derived from Hematopoietic Stem Cell (HSC) of myeloid progenitor lineage.
They have membrane-bound IgG and IgE receptors.
Differentiation and development of eosinophil from a myeloid progenitor in the bone marrow are governed by SCF, IL-3, IL-4, GM-CSF, and CCL11.
IL-5 and CCL11 act in coordination for final differentiation, maturation, and release of eosinophil into the bloodstream.
The granular contents are released during the immune response.
Eosinophil Granular Content
The cytoplasmic granule of the eosinophil is the key enzyme responsible for the functioning of eosinophil. The granule contains the following:
1. Mazor Basic Protein- Also known as MBP, is a protein that induces the mast cell as well as basophil degranulation. These proteins are also involved in peripheral nerve remodelling.
2. Eosinophil Cationic Protein- These are the proteins (ECP) that are responsible for creating pores in the membrane of the pathogen. The pore formation is important as it allows the cytotoxic molecule of the host to enter the pathogen’s body to cause cell lysis by membrane disintegration. It induces fibroblast to secrete glycosaminoglycan. It also suppresses antibody product and T-cell proliferation
3. Eosinophil Peroxidase- Also known as (ECP), it promotes the formation of reactive oxygen species and reactive nitrogen species intermediates. The formation of these compounds in the pathogen body creates oxidative stress in the pathogen which leads to cell death by apoptosis and necrosis.
4. Eosinophil-Derived Neurotoxin - These are RNAse with antiviral activity. They generally shut the transcription machinery of the pathogen.
Functions of Eosinophil
There are the following functions of the eosinophil cell,
They act to remove fibrin during inflammation.
They are involved in neoplasia
They are posted pubertal growth of the mammary gland
They are involved in asthma, an increase in absolute eosinophil count is the major cause of asthma
Absolute eosinophil count is used for the diagnosis of diseases such as Cushing’s syndrome, eczema, and leukemia.
They are involved in inflammatory response
They also act as the antigen-presenting cell to T- cell
They are involved in allograft rejection during raft transplantation.
They are major effector cell, the effector function includes the production of the following compounds-
They produce the following cytokines IL-1, IL-2, IL-4, IL-5, IL-6, IL-8, IL-13.
They are involved in the production of TNF alpha (Tumor Necrosis Factor)
They produce growth factors
They produce elastase
They are also involved in the production of lipid mediators like eicosanoids from the leukotriene (e.g., LTC4, LTD4, LTE4) and prostaglandin.
Eosinophilia
It is a condition in which the absolute eosinophil count in blood is higher than normal conditions.
Absolute eosinophil count is a blood test in which the concentration of a cell is measured. A person has more than 500 eosinophils per microliter, the person is diagnosed with eosinophilia.
Increased eosinophil number can be an indicator of the following conditions like Acute hypereosinophilic syndrome, eczema (skin disorder, itchy or inflamed skin), asthma, or hay fever, it also a sign of various autoimmune disorders. Eosinophilia is also an indicator of leukemia, it is also used as a diagnosis for the early stages of Cushing’s disease, it is a disorder where the cortisol hormone concentration is higher than normal.
Eosinophils’ low count or low concentration can be attributed as the result of intoxication from alcohol. If a person is suffering from Cushing’s syndrome they too have abnormally low counts of eosinophil. Generally, low levels of eosinophils are not considered life-threatening conditions unless all the other white cell counts are also abnormally low. Such conditions generally arise due to abnormality or disorder in the bone marrow.
Eosinophilic conditions in the body include eosinophilic esophagitis, a disorder of the alimentary canal specifically the esophagus, this is generally referred to as esophageal eosinophilia. Another condition is eosinophilic colitis, which is the eosinophilia of the large intestine. It can occur in various organs but these are the most common that suffer from eosinophilia.
Interpretation of Test Result
The normal concentration of eosinophilia in the body is about 0.0 to 6.0%. The absolute eosinophil count can be calculated by the eosinophil number in blood multiplied by the total white blood cell count. On average the normal concentration range is from 30- 35. If the concentration is above 500, an eosinophilic condition is diagnosed. It is also categorized as mild, moderate, and severe. It has a concentration range from 500–1,500 eosinophil cells per microliter for mild, moderate has a range of 1,500 to 5,000 eosinophil cells per microliter, and severe greater than 5,000 eosinophil cells per microliter.
FAQs on Eosinophils: Structure, Functions & Importance
1. What are eosinophils and what is their main role in the human body?
Eosinophils are a specialised type of white blood cell and a component of the immune system. They are classified as granulocytes because their cytoplasm contains large granules. Their primary roles include combating parasitic infections, particularly against helminths (worms), and playing a central part in modulating allergic reactions and inflammation.
2. Can you describe the key structural features of an eosinophil?
An eosinophil is characterised by a few distinct structural features visible under a microscope:
- Nucleus: It typically has a bi-lobed nucleus, where two lobes are connected by a thin strand of chromatin.
- Granules: The cytoplasm is filled with large, spherical granules that are acidophilic (acid-loving). These granules contain enzymes and toxic proteins used to fight pathogens.
- Staining: When stained with an acidic dye like eosin, these granules turn a characteristic bright red or pink, which gives the cell its name.
3. How do eosinophils specifically fight off parasitic infections?
Eosinophils combat parasites through a mechanism different from the engulfing action (phagocytosis) of other white blood cells. They attach themselves to the surface of large parasites, such as worms, that are too big to be ingested. Upon attachment, they release the toxic contents of their granules directly onto the parasite's surface. These contents, including major basic protein (MBP) and eosinophil peroxidase, damage the parasite's outer membrane, ultimately killing it.
4. What is the connection between eosinophils and allergic conditions like asthma?
In allergic reactions, eosinophils are key players in the late-phase response. When an allergen enters the body, other immune cells trigger the release of signals that recruit eosinophils to the site of the reaction (e.g., the airways in asthma). While they play a role in modulating the immune response, an overaccumulation of eosinophils can be harmful. They release inflammatory substances that can cause tissue damage and contribute to symptoms like airway inflammation and constriction, which are hallmarks of asthma.
5. What is considered a normal range for eosinophils in a blood test?
A normal eosinophil count can vary slightly depending on the laboratory, but it is typically reported in two ways:
- Relative Count: Eosinophils usually make up 1-6% of the total white blood cell count.
- Absolute Count: The normal absolute eosinophil count is generally between 30 and 350 cells per microliter of blood.
6. What are the most common causes of a high eosinophil count (eosinophilia)?
Eosinophilia, or a high eosinophil count, is most often caused by the body's response to specific triggers. The primary causes include:
- Allergic reactions: Conditions such as asthma, hay fever (allergic rhinitis), and eczema.
- Parasitic infections: Especially infections by worms like hookworms or roundworms.
- Certain skin diseases: Such as dermatitis or pemphigus.
- Autoimmune conditions: Some autoimmune diseases can trigger an increase in eosinophils.
- Certain cancers: Though less common, some cancers like Hodgkin's lymphoma can cause eosinophilia.
7. While high eosinophil levels are a concern, can low levels (eosinopenia) also indicate health problems?
Yes, an abnormally low count of eosinophils, known as eosinopenia, can also be a sign of certain health issues. It is often less specific than a high count but can be caused by:
- Acute bacterial or viral infections: The body may prioritise producing neutrophils, leading to a temporary drop in other WBCs.
- High stress levels: Production of stress hormones like cortisol can suppress eosinophil levels.
- Cushing's syndrome: A condition involving excessive cortisol production.
- Medications: Use of corticosteroids, which are anti-inflammatory drugs that suppress the immune system.
8. How do eosinophils differ in structure and function from other granulocytes like neutrophils and basophils?
Eosinophils, neutrophils, and basophils are all granulocytes but have distinct differences:
- Eosinophils: Have a bi-lobed nucleus and large, red-staining granules. Their main function is fighting parasites and mediating allergic reactions.
- Neutrophils: Have a multi-lobed nucleus (3-5 lobes) and fine, pale lilac or neutral-staining granules. They are the most numerous WBCs and primarily fight bacterial infections through phagocytosis.
- Basophils: Have a bi-lobed or S-shaped nucleus often obscured by large, coarse, blue/purple-staining granules. They are the least common granulocyte and release histamine and heparin during intense allergic responses.
