2016 Group 7 Project

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Histological images of an eosinophils. Note the well-defined, bi-lobed nucleus and heavily granulated cytoplasm.

Eosinophils are a type of leukocyte or white blood cell which has a role in the immune system in regulating an inflammatory response during infections, injuries, allergies and in the presence of tumours[1]. In a healthy body, eosinophils normally account for 1-3% of all leukocytes in peripheral circulation, with approximately 30-350 cells per cubic mm of blood[2]. This means most eosinophils can be found in tissues with only around 1/100 existing in the circulatory system[3]. Maintaining this ratio via activation and cell death is important for maintaining a healthy immune system[4]. In the case that there are infections, allergies, etc. the level of eosinophils present in the blood stream can increase to be more than 4.5x108/L (450μl)[5].

Eosinophils mature in the bone marrow via hematopoietic progenitor cells[6]. They grow to be 10-12 µm in diameter and develop a bi-lobed or ‘kidney shaped’ nucleus which makes it easily distinguishable[7]. They are also characterised by the presence of granules which can be classified into specific granuels, which occupies most of the cytoplasm, and primary granules[8]. The eosinophil’s cytoplasm also contains lipid bodies, which generates eicosanoids[8][9], and sombrero vesicles which provide alternative secretory pathways to granules and have the appearance of a flattened tubule[10][11].

History of Discovery

Year Finding
1590 Compound microscope invented by Hans and Zacharias Janssen, making it possible to observe the components of blood[12].
1658 Jan Swammerdam (1637 -1680) first observed and identified red blood cells[13].
1843 French professor, Gabriel Andrai (1797–1878) and English practitioner, William Addison (1802–1881) simultaneously described leukocytes. They recognised that both red and white blood cells were altered during disease. Addison also postulated that pus cells were circulating leukocytes that had crossed the walls of blood vessels[12][13].

Gottlieb Gluge (1812–1898), a Belgium clinician and scientist published his major work, ‘Atlas der Pathologischen Anatomie’ describing ‘granule cells’ which he referred to as ‘compound inflammatory globules’. These cells were not only observed in inflammatory exudates (pus and serum) but also in colostrum and the ovaries[13].

German Pathologist Julius Vogel (1814–1880) published his revolutionary book. ‘Pathological Anatomy of the Human Body which contained various macroscopic and microscopic illustrations, including granular cells in inflammatory exudates. These depicted cells with the morphological characteristics of eosinophils, both intact and at different stages of degranulation[13].

1846 British physiologist and ophthalmologist, Thomas Wharton Jones (1808–1891) described ‘finely granular and coarse granular blood cells’ in numerous species including the human, lamprey, fowl, frog, horse and elephant. The coarsely granular white blood cells demonstrated the distinctive appearance of eosinophils and Jones estimated the diameter of their granules to be approximately 1 micrometre[13].
1865 Max Johann Sigismund Schultze (1825–1874) first described four different types of leukocytes (now identified as monocytes, lymphocytes, neutrophils and eosinophils). He used warm stage microscopy to closely observe fine and coarse granular white blood cells and determined that these cells moved in an amoeboid like fashion and phagocytosed small particles[13].
1874 Heinrich Caro (1834–1910) discovered eosin, a fluorescent red dye formed by the addition of bromine to fluorescein. It is acidic and therefore stains basic proteins[13].
1879 Paul Ehrlich referred to the eosinophil for the first time. He developed a technique for staining blood films, which involved a simple heating and air-drying process to fix the samples and the addition of acidic (eosin) or basic coal tar dyes. This not only lead to the discovery of eosinophils, but neutrophils, lymphocytes and basophils as well. Ehrlich examined the distribution of eosinophils in tissues and described numerous features of granules (alpha granules)[13]. He noted their round or rod shaped appearance and correctly deduced that the granules contained secretory components. He also noted the variation in the number of granules and nuclear lobes from cell to cell. Ehrlich observed idulin (black) staining beta-granules in (eosinophil) myelocytes, likely to be immature crystalloid granules. He identified asthma, skin diseases, helminths and medication reactions as causes of eosinophilia[13].
1990 Antiviral activity of eosinophilic granule proteins discovered[14].

Birth, Life and Death in the Body

Eosinophil Lineage


Differentiation of hematopoietic stem cells occurs in the bone marrow in response to growth factors produced by resident stromal cells and locally elevated cytokines. Hematopoietic progenitors are undifferentiated multipotent stem cells, meaning they can develop into all blood cell types and possess self-renewal capacities (by division)[15]. These are identified by the presence of CD34 on their surface, a marker highly expressed on progenitor cells. These multipotent progenitor cells further to differentiate into either a common lymphoid progenitor or a common myeloid progenitor which then differentiate into downstream progenitors before terminal differentiation into a specific blood cell[15].

The eosinophil lineage-committed progenitor (EoP) is derived from a common myeloid progenitor. These are distinguished by surface markers, CD34, IL-5 receptor alpha (IL-5Rα) and CD117 (lower levels)[16]. EoPs mature into eosinophil precursors (preEos), like eosinophils, these contain granules however still possess the ability to proliferate. Upon stimulation with specific cytokines, a preEos will mature into an eosinophil (terminally differentiated and no proliferative capacity)[16]. Cytokines IL-3, IL-5 and GM-CSF induce eosinophil maturation and formation from hematopoietic progenitors. IL-5 particularly directs differentiation and encourages proliferation of EoPs and preEos cells[8][16].

Presence in Circulation and Migration to Tissues

Multistep process of eosinophil trafficking[17]

Following maturation and/or activation, eosinophils (as well as their progenitors) are mobilised, released from bone marrow into circulation and trafficked to tissue sites. A large proportion of these mature eosinophils will remain in bone marrow[8][18].

Once eosinophils enter circulation, they have a half-life of approximately 8–18 hours[8]. Under normal conditions, the vast majority of eosinophils are located in tissues (the tissue/blood eosinophil ratio is about 100:1) and upon gaining entrance to a tissue, most do not recirculate[3]. They have a life span ranging from 2 to 5 days, however locally produced cytokines such as IL-5, IL-3, GM-CSF, IL-33, and interferon-γ may increase this survival time (up to 12 days)[3][8][18]. Eosinophils are predominantly trafficked to mucosal surfaces of the respiratory, lower genitourinary and gastrointestinal tracts where they reside within the lamina propria (excluding the oesophagus). They are also localised within the thymus (medulla and junction between the medulla and cortex), mammary glands, ovaries, uterus, spleen and lymph nodes as well as inflammation sites involving Th2 cells. During allergic reactions, they can be found also be found in the skin [3][8][19].

The recruitment of eosinophils, involves integrin-mediated adhesion, signalling through extracellular matrix components, cytokines and chemotactic molecules[20]. This process is primarily regulated by IL-5 as well as the chemokines, eotaxin-1, -2 and -3 which are all CCR3 ligands (Note: the CCR3 receptor is present on eosinophils). Type 2 cytokines produced by myeloid dendritic cells and NK cells (e.g. IL-4, IL-13 and IL-9) also promote eosinophilia by upregulating the expression of adhesion molecules such as Vascular cell adhesion protein 1 (VCAM-1)[20][8][3]. Eosinophils express both α4β1 and α4β7 integrins as well as P-selectin glycoprotein ligand 1 (PSGL-1). These enable cells to roll and adhere to the endothelial cells expressing P-selectin and VCAM-1 or mucosal addressin 1. These are important interactions that permit transendothelial migration into tissue[20]. Other important chemotactic molecules involved in this process include platelet activating factor (PAF), leukotrienes (leukotriene B4) and prostaglandins (prostaglandin D2)[20].


Eosinophils are potent inflammatory cells, they carry out antigen presentation and are involved in exaggerating inflammatory responses through the synthesis and release of lipid mediators and cytokines[21]. They are associated with the pathogenesis of allergic diseases, parasitic, helminthic, bacterial and viral infections, injury and tumour immunity[1]. Impaired apoptosis and removal of apoptotic cells (by phagocytes) can lead to chronic inflammation, hence modulating eosinophil activation and apoptosis is critical in avoiding excessive damage to tissues and resolving inflammation[4][21].

Reactive oxygen species (ROS) are known to regulate the survival of leukocytes during inflammation. These include hydrogen peroxide (H2O2), superoxide O2-, hydroxyl radical (OH) and nitric oxide (NO). H2O2 in particular has been shown to promote resolution of inflammation in allergic disease, this is achieved through the initiation of caspase-dependent apoptosis[21][22].A study demonstrated that natural killer cells (NK lymphocytes) also possess inhibitory effects on eosinophils, they were shown induce eosinophilic degranulation and apoptosis (via direct cell-cell contact). Eosinophil degranulation seemed to be dependent on mitogen-activated protein kinase (MAPK) and PI3K pathways, whilst apoptosis was dependent on mitochondrial ROS[22].


Characteristics of Eosinophils[23]


Eosinophils average 10-12 µm in diameter and possess a distinct bi-lobed nucleus, with highly condensed chromatin (nucleoli are not visible)[7]. The two nuclear lobes are of equal size and are usually well demarcated, this has no obvious discernible function however is a useful indicator of Eosinophils in blood smears.

Primary and Specific Granules

Eosinophils are characterised by the presence of granules, there are two major types, specific granules (found only in eosinophils) and primary granules (these are similar to granules found in other granulocytes)[8]. Specific granules occupy most of the cytoplasm; they are relatively large (~0.5-1 µm in diameter), round vesicles and contain a multitude of cytokines and chemokines as well as basic cationic proteins that give eosinophils their distinctive staining properties (red or pink appearance when stained with eosin or other similar acidic dyes)[8][24][25].

Smear showing hexagonal Charcot Leyden crystals in a background of inflammatory cells and necrotic material [26]
Primary granules form early in eosinophil development and are enriched with Charcot-Leyden crystal proteins[8]. They are one of the most abundant proteins in eosinophils (7-10%) and form colourless crystals that are usually 2-4 µm in diameter and appear hexagonal or bi-pyramidal in shape[27][28]. They have been found in body fluids, tissues and secretions thus have been considered a hallmark of eosinophil involvement in allergic reactions and other inflammatory reactions such as asthma or vasculitis. It has also been noted that CLC proteins persist long after eosinophil have died. However, their role in eosinophil function or inflammation is yet to be revealed[27][28].

CLC proteins have been identified as members of the carbohydrate-binding family of galectins (Galectin-10)[27]. They lack secretory signal peptides, suggesting they primarily function intracellularly, however their exportation and secretion also provides evidence for an extracellular function as well. Studies have also noted their interaction with cationic proteins and involvement in the secretion of EDN and ECP specifically. It is known that CLC proteins have a role in immunoregulation and immunomodulation in regulatory T cells (t regs). Eosinophils also possess these abilities, hence it has been stipulated that the protein may play a part in regulating the proliferation and function of CD4+ cells[27][28][29].

Lipid Bodies

Eosinophils also possess lipid bodies, these are lipid-rich cytoplasmic inclusions that form rapidly following eosinophil activation. These cytoplasmic structures are not membrane bound, they contain eicosanoid synthetic enzymes (5-LO, LTC4 synthase, COX) which act to generate important eicosanoids such as Leukotrienes, Prostaglandins and lipoxins[8][9]. Eicosanoids are biologically active, oxygenated fatty acids that behave as paracrine mediators of inflammation (act on nearby cells to stimulate the inflammatory response) in addition to intracellular signals. They have been identified as key molecules in the pathogenesis of many inflammatory diseases such as psoriasis and rheumatoid arthritis[9].

Eosinophil Sombrero Vesicles

File:Eosinophil sombrero vesicles (EoSVs).jpg
Ultrastructure of isolated eosinophil granules and eosinophil sombrero vesicles (EoSVs)[30]

Large vesiculotubular carriers called eosinophil sombrero vesicles (EoSVs) exist within the cytoplasm of eosinophils. These are flattened, folded and elongated tubules with relatively large membrane surface areas and a diameter of 150–300 nm[10][11]. EoSVs offer a different secretory pathway in which they rapidly transport products (cytokines and cationic proteins) from granules to the plasma membrane for extracellular release during the absence of granule-granule or granule-plasma membrane fusion[10]. This process occurs upon stimulation of eosinophils with the typical agonists (e.g. MBP has been found in vesicles of eosinophils stimulated by eotaxin) and is characteristic of eosinophils undergoing piecemeal degranulation. EoSVs arise from specific granules and have been observed budding from emptying granules[10][31]. They are also released from eosinophils undergoing cytolysis where they are deposited within target tissues together with free granules. Furthermore, EoSVs are present in non-stimulated eosinophils (with or without granule products)[10][31].


What are they?

Granules are trilaminar membrane bound organelles with a matrix surrounding an electron dense crystalline core composed of MBP-1 and MBP-2. The granules contain highly structured internal membranes that serve to compartmentalize the granule. It has been portrayed with TEM and electron tomography studies that following agonist activation these compartments rearranged themselves. Suggesting that different stimuli can lead to the contents of the granules to be segregated, sorted and selectively secreted from the organelle.[32][33][34] a

Transmission Electron Microscope Image of an Eosinophil and its Granules


Granula Contents Function
Major Basic Protein (MBP)
  • Forms crystalloid structure of granules
  • Toxic to helminthic worms[35]
  • Cytotoxic to airways (partially responsible for tissue damage in asthma)[36]
Eosinophil Cationic Protein (ECP)
  • Creates voltage-insensitive, ion-selective toxic pores in the membranes of target cells (to allow other cytotoxins to enter)[37]
  • Ribonuclease activity (but not as strong as EDN)[38]
  • Suppression of T cell proliferative responses[39]
  • Immunoglobulin synthesis by B cells[39]
  • Mast cell degranulation stimulation of airway mucus secretion[39]
  • Glycosaminoglycan production by human fibroblasts[39]
Eosinophil Perocidase (EPO)
  • Catalyze the oxidation of halides, pseudohalides, and nitric oxide to form highly reactive oxygen species which go on to oxidise nucleophil species on proteins causing oxidative stress on the host cell, leading to apoptosis and cell death[40]
Eosinophil-Derived Neurotoxins (EDN)
  • Ribonuclease activity
  • Anti-viral activity[41]
  • Induce migration and maturation of dendritic cells[42]
Mechanisms of granule secretion[31]

Mechanisms for contents release

  • Classic Exocytosis
    Eosinophil in PMD. B: Full granule C: Partial granule release D: Full granule release

When a single granule fuses with the plasma membrane of the cell, releasing its contents extracellularly[31].

  • Compound Exocytosis

Where more than one granule is released extracellularly when the granules first fuse with each other then the plasma membrane. Mainly used to combat big targets such as helminthic (worm) parasites[32].

  • Piecemeal Degranulation (Predominant Method)

Process where the contents of the granules are selectively extracted into tubular or spherical vesicles which then travel through the cytoplasm to the plasma membrane where they fuse and release their contents[43][44].

  • Eosinophil Cytolysis
    Eosinophil undergoing cytolysis. Note the free intact extracellular granules

Following lysis of the cell and loss of the plasma membrane, membrane bound granules are released freely into the surrounding tissue. This is seen mostly in eopsinophilic disorders, such as being found in the sputum of asthma sufferers[45][46].

Surface Markers

Eosinophils migrating to different tissues in the body are part of its function[47]. Eosinophils that are part of the circulatory system remain inactive until they reach the tissue[48]. When eosinophils migrate to endothelial cells, interleukin (IL)-4 or IL-Beta encouragse further migration[48]. The rate of this process further increases if a chemoattractant is used[48]. In an experiment where a culture is used, the endothelial cells that were treated to prevent this chemotactic event lead to a decrease in the expression of CD68[48]. CD69 is an early marker and CD35 is a receptor[49]. Both of these are controlled by endothelial cells and thus their expression increased when the eosinophils migrated to the endothelial cells[49].

Granules express receptors for cytokines and G protein coupled receptors (CCR3) for chemokines. These are located on their surface membranes and respond to external cytokines and chemokines by activating a signal-transduction pathway within. IFN-γ (cytokine) and eotaxin (chemokine) are primarily responsible for stimulating secretion of the cationic proteins, enzymes and cytokines originating from granules[50]. Granules essentially function as individual secretory vessels outside of eosinophils in diseased tissue sites, this reveals how they may contribute to inflammation mediated by eosinophils as well as immunoregulation and immunomodulation[50].


Large influxes of eosinophils from circulation occur mostly in sites during an infection or an inflammatory response[51]. These cells are released into the blood stream before migrating to the part of the tissue that is inflammed. This action is dependent two main factors. One being the relationship between the adhesion molecules, that are found on eosinophils, and the endothelium and the second being the chemotactic signals generated by cytokines, chemokines and chemoattractant receptors[52][53]. Some of the chemotactic molecules include FMLP, platelet-activating factor, complement 5a, and Chemokines. Human chemokines can be classified into CXC, CC, CX3C, and C subfamilies[54][55]. Chemokines bind to G-protein-coupled seven-transmembrane receptors on different leukocytes which results in different cellular events[56]. Humans express four different types of G-protein-coupled chemokine receptors (CCR):

Initially, the cytokines Interleukin (IL) -3 , IL-5 and GM-CSF prime the eosinophil whilst they are still in the bone marrow, before migrating to blood vessels[59]. The transduction pathways of eosinophils then involve CCR1 and CCR3, where CCR3 is utilised for eotaxin signals[60]. From here eosinophils migrate according to the chemotactic gradient caused by chemokines which promotes the adhesion and transmigration process from the peripheral blood vessel to the tissue[61] after chemokine receptors aggregate with the anterior pole of the eosinophil and cause it to polarise[62].



In the context of response to allergic challenges in the airways of mice, eosinophils have demonstrated the ability to activate naïve T cells by acting as antigen presenting cells (APCs) in contrast to their normal role as end-stage effector cells that release granular cationic proteins[63]. With the increasing prevalence of asthma, research into the role of eosinophils in the bronchial inflammatory response has increased, with the release of IL-5, eotaxin and other eosinophilic cytokines and chemokines being seen as the target for novel chemotherapeutic agents due to the damage that they cause to the bronchial epithelium[64]. Allergic challenge stimulates the eosinophils to release IL-5 and granulocyte macrophage-colony-stimulating factor (GM-CSF), both of which are implicated in the inflammatory response characteristic of asthma[65]. The release of toxic eosinophilic cationic proteins (ECP) allows the detection of inflammation in rhinitis by correlating serum ECP with that in nasal fluid, differentiating between systemic and local pathology[66].



Studying the role eosinophils play in helminth infection is quite difficult, mainly due to the fact that humans and rodents don’t share many common helminthic worms making interpretations from rat models difficult. They is some evidence that they have a role for IL-5 (eosinophil activator) in protective immunity[67], however, other IL-5 receptor cells such as B-cells and basophils have not been ruled out in that model[68]. However it has been shown that during parasitic infection, the granules such as eosinophil peroxides (EPO) and major basic protein-1 (MBP-1) eosinophil-derived neurotoxin and eosinophil catatonic protein can deposit its contents onto the helminth to kill it[69]. In vitro investigation into the mechanism whereby eosinophils adhere and degranulate when attacking Schistosoma Mansoni revealed that there are two distinct steps; firstly weak adhesion is mediated by IgG receptors and then the degranulation process itself provides strong and irreversible binding to the schistosomula's membrane[70]. By using murine hosts, Muniz et al. found that the purinergic P2Y12 receptor (P2Y12R) played a key role in the host inflammatory response to the S. Mansoni infection, ultimately triggering eosinophilic granulation[71]. Thus eosinophils are needed for eliminating the the parasite as without it, the parasite would be able to survive in the body for a longer period of time[72].

Time Lapse Video of Eosinophils Attacking a Helminth:

YouTube Link


File:Rsv extravasion.png
Extravasion of eosinophils in RSV infection[73].
Using transgenic mouse strains infected with a pneumonia virus of mice (PVM), Lacy found significantly increased survival rates and lower pulmonary viral loading when eosinophilic degranulation was not supressed[14]. Further research using PVM confirmed that eosinophilic granular proteins (ECP and EDN) bound to and degraded single stranded RNA containing viruses, such as the respiratory syncytial virus[74]. Paradoxically, they have also been shown to be susceptible to HIV-1 infection in vitro so may be a reservoir for the virus in vivo[75].


Through release of their cytotoxic granule proteins (EDN and MBP) in the extracellular matrix, eosinophils have demonstrated a role in fungal infection fighting, as the cytotoxins, upon contact, can kill fungal organisms[41]. Yoon et al. (2008) showed that through β2 integrin protein, the eosinophils were able to adhere to and react to the β-glucan found in the cells walls of the fungus Alternaria alternata, prompting the Eosinophil to release EDN, compromising the integrity of the fungus’ cell wall[76]. It is interesting that eosinophils reacted to non-pathogenic fungi that were ignored by neutrophils, and it is posited that this process may be related to the presence of other infective agents or antigens[77].


Upon exposure to bacteria, eosinophils readily release mitochondrial DNA and both ECP and MBP, together they form extracellular structures like traps. These traps then bind to and kill the bacteria[78].
Skin lesions on the inside of the elbow of an Atopic Dermatitis sufferer

Role in Allergy and Disease

Atopic Dermatitis (Eczema)

An elevated level of ECP in the peripheral blood seems to correlate with the level of disease activity in Atopic Dermatitis (AD) patients. More specifically, the cytotoxic granule proteins from eosinophils are deposited within the skin lesions[41]. Even in the absence of eosinophils, Davis et al, (2003) showed high levels of positive staining for MBP in the skin lesions[79]. Eosinophilia has been demonstrated as being highly predictive of food allergies in AD sufferers, with food allergy being shown as causative of AD symptom aggravation in greater than 70% of the patients studied[80]. Additional linkages between AD and food allergies are discussed by Majamaa et al.[81], with faecal ECP and Tumour Necrosis Factor alpha (TNF-α) concentrations increasing after ingestion of cow's milk in children with AD ezcema. These findings are highly suggestive that eosinophils play a significant role in the pathogenesis of AD.


IL-5 and eotaxin-induced eosinophil recruitment in allergic asthma[82]
Significant amounts of MBP have been found in the bronchoalveolar lavage fluid of patients with asthma, enough to induce cytotoxicity in host tissues such as epithelial cells as well as increase smooth muscle activity, therefore indirectly causing airway hyperactivity[83][84]. In addition, the presence of eosinophils in respiratory tissue may lead to increased vascular permeability, potent smooth muscle constrictions and mucus secretion due to their generation of cysteinyl leukotrienes[85]. Therapeutically, it has been found to be beneficial to treat allergic airway disease through cysteinyl leukotriene inhibitors.

Half of all asthma patients are affected by eosinophilic asthma[86]. The disease is mainly controlled by T-helper cells with the assistance of eosinophils[87]. The disease can evolve to slowly change the structure of the respiratory tract[87]. Eosinophilic asthma can be monitored to prevent the worsening of the condidtion via:

1. Monitoring levels of sputum eosinophils[88]

2. Looking into the blood eosinophil count[89]

Sputum eosinophils are directly related to eosinophilic asthma as they control inflammation[86].

Eosinophilic Oesophagitis, Gastritis, and Gastroenteritis

Inflammation of the oesophagus and gastrointestinal tract due to mass migration of eosinophils into these tissues in response to an allergen, leading to difficulty swallowing and discomfort in digestion[41]. Interestingly, it has been found by Akei et al. (2006) that in rat models, rats with AD showed eosinophilic oesophagitis in response to an allergen with migration of eosinophils to the tissue, where as rats without AD did not. This suggests a co-pathogenesis of eosinophilic inflammation in the skin and eosinophilic oesophagitis[90]. In line with other eosinophilically mediated disorders, eosinophilic oesophagitis demonstrates a significant linkage to allergic phenomena, with food allergies predominating[91][92]. In contrast,eosinophilic gastritis and gastroenteritis occur but the linkage to allergic causation is significantly weaker and it is seen as more likely that these conditions are better thought of as sequalae of generalised eosinophilia[93][94][95].

Eosinophils and Vasculitis

Schematic representation of eosinophil trafficking[23]

The role played by eosinophils in vasculitis has mainly been examined in conjunction with eosinophilic granulomatosis with polyangiitis (EGPA), which was renamed from Churg–Strauss Syndrome in 2012 to better reflect the disease pathology[23]. Eosinophils have been implicated in systemic vasculitis, with a significant linkage demonstrated between eosinophilia and asthma[96] as well as other allergic conditions. Eosinophilic vasculitis appears to be connected to connective tissue disease, with a similar pathogenesis to vasculitis being demonstrated by the presence of ECP in the biopsied connective tissue lesions[97]. Khoury et al.[23] discusses the connection between eosinophilic granular chemokines and EGPA, additionally showing robust linkages between these biomarkers, eosinophilia and asthma. Due to the circulatory system being integral to all major organ system,,eosiniphilic vasculitis can have manifest impacts almost anywhere in the body, with the central nervous system[98][99], bronchial tract[100] and gastrointestinal tract[101] being more commonly affected due to their high vascularity.


Within all circulating leukocytes, less than 7% are eosinophils. In a normal patient, the eosinophil blood count would be under 4.5x108/L (450μl), however if the eosinophil blood count is greater then 4.5x108/L (450μl) in the peripheral blood. Diagnosis of eosinophilia is completed by a complete blood count (CBC) while in certain circumstances an absolute eosinophil count is conducted. The exceeding eosinophils can be found either in the bloodstream or directly found in tissues. An increase in eosinophils is commonly just a secondary side effect of another disease or in rare cases it by may be via a primary disease (idiopathic).

Haemolymphatic neoplastic disease has the potential to elicit eosinophilia as do a number of conditions including drug reactions[102] and autoimmune disorders such as systemic lupus erythematosus[103]. Drug reaction with eosinophilia and systemic symptoms (DRESS) is a well documented cause of eosinophilia[104][105][106][107] that is characterised by a 2-3 week delay between commencement of medication and the appearance of symptoms and flare-ups can continue even after discontinuation of therapy[107]. DRESS is considered to be a hypersensitivity type reaction caused by a relatively small number of substances[105] and there is some controversy surrounding its definitive dia=gnosis and treatment[104].

Once the complete and absolute blood counts are completed, the underlying cause can be identified and is targeted in variety of treatments used to lower the eosinophil levels[5].

  • Corticosteroids - eosinophil survival and stimulates eosinophil clearance from tissues
  • Imatinib - Inhibits receptor tyrosine kinase activity
  • Hydroxyurea - Cytotoxic agent
  • Interferon-α - Inhibits eosinophil growth and functional responses

Additionally, after treating the underlying cause of eosinophilia, immunomdulation is being seen as the main tool for mitigating systemic effects[108][109], with a number of drugs used primarily to treat neoplastic conditions being used in lower doses[109][110].

Respiratory Syncytial Virus

Respiratory Syncytial Virus is considered to be one of the most significant respiratory pathogens known worldwide, with infection ensuing in a bronchiolitis affecting the pediatric age group (0 to 3 years)as well as the institutionalized elderly. Many studies have shown that eosinophils have implicated results in the pathogenesis of Respiratory Syncytial Virus. Eosinophils are recruited towards the infection site and degranulate into the lung parenchyma in conjunction with severe Respiratory Syncytial Virus infection, and RSV-infected epithelial cells produce and realease a variety of potential eosinophil chemoattractants[49][50]. These results stand to portray eosinophils as the villians of RSV disease, while most studies have focused on the role of eosinophils in promoting tissue damage and bronchospasm From a different perspective, studies consider the possibility that eosinophils may also have a positive role to play and that eosinophilic inflammation associated with RSV disease may actually represent more of a “double-edged sword,”[49][50].

The double-edged sword, reflecting the opposing beneficial and harmful features of a given physiologic response, is an idea that has successfully taken hold in our understanding of the physiology of the neutrophil and of diseases involving dysregulation of neutrophil conscription and activation. For example, although neutrophils are evidently effective at mediating host defence against fungal and bacterial pathogens via phagocytosis, degranulation, and manufacture of reactive oxygen species, the dysregulation of these essential, positive responses can lead to reperfusion injury and adult respiratory distress syndrome (RDS)[49][50]. The same can be said for eosinophils and the pathogenesis of reactive airways disease. However, there are studies that have proposed a correlation connecting severe Respiratory Syncytial Virus infection in infancy and the growth of reactive airways disease in later childhood years. Even without a direct link to this precise virus infection, respiratory virus infections are adequately commonplace so as to be considered a universal affliction of infancy and childhood.[49][50].


Term Definition
Eosinophil A white blood cell that is accountable for combating multi cellular parasites and infections.
Granulocytic Effector Cells Are cells that act in defence against helminth infections.
Leukocytes Circulating cells in the blood which act as a defence against foreign material and diseases, e.g. monocytes, granulocytes and lymphocytes
Inflammatory Exudates Is a fluid that filters via the circulatory system into lesion or regions of inflammation, can either be transparent fluids or pus filled.
Amoeboid Is a common form of locomotion in eukaryotic cells
Hematopoietic Cell Are stem cells in which give rise to all other known blood cells via haematopoiesis.
Proliferation A rapid multification of parts or a rapid increase in the number of cellular components
CD34 Is a cluster of diversity in a cell surface glycoprotein and its roles is to act as a cell to cell adhesion factor.
IL-5R Interleukin-5 receptor is a type 1 cytokine receptor.
CCR3 C to C chemokine type 3 receptor.
Chemokines A group of small cytokines/signalling proteins.
IL-5 Interleukin 5 is produced by Type 2 T helper cells/mast cells.
IL-3 Improves the body’s natural response to a disease.
GM-CSF Granulocyte macrophage colony-stimulator factor is a monomeric glycoprotein that is secreted by a variety of cells such as mast cells, T cells, Macrophages, NK cells, Endothelial cells and Fibroblasts
IL-33 Increases the production of T helper-2 cytokines.
Interferon-γ A dimerised soluble cytokine, that is important for innate and adaptive immunity against viral infections and also some protozoal and bacterial infections.
IL-13 Induces differentiation of helper T cells to T Helper 2 cells.
IL-4 A class of proteins which have carbohydrate groups attached to the polypeptide chain.
Eotaxin 1 Known as CCL11 (C-C chemokine 11) – Recruits eosinophils by preventing their chemotaxis in order to be implicated in allergic responses.
VCAM-1 Vascular Cell Adhesion Molecule 1 mediates the adhesion of lymphocytes, monocytes, basophils and eosinophils to vascular endothelium.
CCL5 Chemokine ligand 5 is chemotactic for eosinophils, basophils and T cells while it also recruites leukocytes into inflammatory areas.
Apoptosis Is a process of cell death.
Charcot-Leyden Crystal Proteins Are microscopic crystals that are found in patients with allergic disease such as asthma, pneumonia or ascariasis.
Vasculitis Inflammation of a blood vessels or multiple blood vessels
Galectins A group of proteins that bind specifically to β-galactoside sugars.
Immunoregulation The control of immune responses and interactions between B and T lymphocytes and macrophages.
Immunomodulation A process in which an immune response is altered to a preferred level.
ECP Eosinophil Cationic Protein is a released during degranulation in eosinophils.
CD4+ Is a glycoprotein found on the surface of T helper cells, Monocytes, Macrophages and Dendritic cells.
Eicosanoids Are signalling molecules.
Leukotrienes Are a group of biologically active compounds, originally isolated from leucocytes (metabolites of arachidonic acid).
Prostaglandins Are a group of compounds that have different hormone like effects (cyclic fatty acids)
Lipoxins Are a group of compounds generated form arachidonic acid.
MBP-1 Mlul-box binding protein is a transcription factor involved in the regulation of cell cycle progression from G1 to S phase.
TEM Transmission elector microscopy is a microscopy technique used to capture images at higher resolutions than light microscopes .
Chemoattractant Are substances that possess chemotaxis inducers in motile cells
Phagocytosed Devour or destroy a bacteria and other foreign material
CD68 Used as a marker for monocytes and macrophages.
CD69 Is induced by the activation of T lymphocytes and NK cells.
Respiratory Syncytial Virus Is a syncytial virus that causes respiratory tract infections.
β2 integrin protein Is a transmembrane receptor that acts as a cell to cell and cell to extracellular matrix interactions.
Polyangiitis Is an autoimmune disease that is characterised by a systemic, pauci-immune, necrotising, small vessel vasculitis.
Idiopathic A disease or condition which arises spontaneously or has an unknown cause.
Corticosteroids A class of steroid hormones that are produced in the adrenal cortex.
Imatinib Is a tyrosine – kinase inhibitor that is used in the treatment of multiple cancers.
Hydroxyurea Antineoplastic drug used in myeloproliferative disorders.
Interferon-α Is a group of interferon proteins that assist in regulating the activity of the immune system.
Dysregulation Is an abnormality/impairment in the regulation of a metabolic, physiological, or psychological process.
Degranulation Is a cellular process that releases antimicrobial cytotoxic molecules from secretory vesicles known as granules found inside cells.
Reactive Oxygen Species Reactive oxygen species (ROS) are chemically reactive molecules containing oxygen.ROS are formed as a natural byproduct of the normal metabolism of oxygen and have important roles in cell signaling and homeostasis.
Reperfusion Injury Is the tissue damage caused when blood supply returns to the tissue after a period of ischemia or lack of oxygen.
Adult Respiratory Distress Syndrome Is a life-threatening lung condition that prevents enough oxygen from getting to the lungs and into the blood.
Reactive Airways Disease Is a term used to describe a history of coughing, wheezing or shortness of breath triggered by infection.


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