2016 Group 5 Project
- 1 Introduction
- 2 History
- 3 Physiology
- 4 Pathology
- 5 Glossary
- 6 References
Mast cells are immune cells that are of haematopoietic lineage traditionally thought to only play a role in pathogen surveillance, however, studies of the interactions between host and parasite have illuminated another role, that being one of protection.  The major role of mast cells in pathogen recognition is supported by their location in host peripheral tissues that commonly come into contact with pathogens, such as skin, mucosal membranes of the respiratory system and even the gastrointestinal tract. Their location at the host-environment interface allows them to proliferate in response to appropriate stimulus and communicate the presence of pathogens to the lymph nodes and other immune cells.   Mast cells are highly granulated which typically contain proteases, particularly tryptase and chymase, that are influenced and regulated by the presence of cell mediators such as Interleukin-4.  Armed with these preformed granules, mast cells can alter their phenotype depending on their environment, demonstrated by selective cytokine production and the altering of transcription processes and storage of preformed mediators.   The presence of preformed granules allows mast cells to respond quickly to pathogen invasion, establishing their major role in the immediate phase of response to allergic pathogens. Mast cells are best known for their role in allergic disease such as asthma, eczema and allergic rhinitis, where degranulation occurs, releasing mediators such as histamine and promoting the acute inflammatory response. 
Mast cells are first identified and named ‘Mastzellen’ by Paul Ehrlich in his doctoral thesis at Leipzig University, Germany. Ehrich described these Mastzellen as being often associated with nerves, blood vessels and glandular ducts. 
German physician Paul Gerson Unna described mast cells in association with pathology. He noted that the cutaneous lesions called urticarial pigmentosa (UP) were accompanied with an increased number of mast cells below the lesions.
Mast cells are thought to originate from myeloid stem cells. 
Erik J. Jorpes identified Mast cells as carriers of heparin. 
JF Riley described Mast cells as containing histamine. 
Mast cells are understood to play an important role in anaphylaxis.
Mast cells are found to be a source of TNF-α. 
Mast cells are seen as contributors to innate immunity.
Mast cells are understood to release growth factors and affect angiogenesis, which contributes to tumour progression.
It is understood that there is a relationship between regulatory T cells and mast cells- T(reg) cells suppress mast cells which is important in allergic reactions 
Evidence that mast cells play a protective role in the growth of intestinal tumours 
Mast cells are seen to have a protective effect in preventing and repairing damage to to the gastric mucosa in peptic ulcer disease. 
Mast cells are shown to have an effect on scar tissue formation. The blocking mast cell activation results in less scar tissue formed. 
The morphology of the mast cell is not always consistent, as they tend to differ greatly in shape depending on where they are situated in the body. For example, when in loose connective tissue they will appear rounded, whereas in dermal fibres they appear spindle shaped, and when in close proximity to blood vessels they can appear elongated.  Because of this, mast cells are divided into sub groups, connective tissue mast cells (CTMC) and mucosal mast cells (MMC).  Mast cells are on average 12-13µm in diameter. Mature mast cell cytoplasm contains up to 1000 granules, which are chemical mediators that range from 0.2-1.3µm in diameter. These granules contain substances such as histamine, proteases, cytokines and growth factors that are enclosed in a heparin proteoglycan matrix. These granules can take up just over half of the cells cytoplasm and contain a double membrane. The structure of these granules varies between human and animal mast cells.  The granules are metachromatic and can be stained with thiazine dyes e.g. toluidine blue because they contain sulphated glycosaminoglycans. When observed under a scanning electron microscope, short and long microvilli can be seen on the surface of mast cells, the granules are visible as well as caveolae- for discharging granules. A centrally located nucleus is also present among the granules that is round or oval in shape and monolobed. Mast cells possess other organelles such as mitochondria, golgi apparatus, endoplasmic reticulum, and ribosomes.  A mast cells' morphology can however be altered according to their environment, disease, stress and even depression can cause changes in shape as well as granule distribution. 
The images on the right show Mast cells and their morphology, image A is stained with toluidine blue.
The function of the mast cell is quite diverse, as they are involved in allergic reactions, innate and acquired immunity, inflammation, tumours, bacterial infections, autoimmunity and tissue repair.  Their functional diversity can be attributed to the vast range of biologically active substances that they can produce, such as heparin, tryptase and chymase, serotonin and dopamine to name a few.  Mast cells are important cells in innate and acquired immunity as they are involved in early recognition of pathogens. 
In regards to the immune system, they play an important role in recruiting other immune cells and controlling the function of immune cells such as T and B lymphocytes. For mast cells to begin to serve a function, they must first become activated. Mast cells become activated in many ways, they can be directly stimulated by pathogens through pattern recognition receptors (PRR), as well as stimulation via the immunoglobulin E (IgE) receptor FcεRI. This eventually leads to degranulation and the release of mediators by the mast cell. Not only do mast cells release chemical mediators, they also have the ability to participate in phagocytosis, and can also produce antimicrobial peptides that have been shown to kill bacteria. Mast cells have the ability to release selective mediators without degranulation, which means that an anaphylactic reaction will not occur.  Mast cells also possess an interesting ability to be triggered by certain molecules and then activate or degrade them. For example, they can synthesis the cytokine endothelin, but can also degrade it. 
Their function in wound healing and tissue repair revolves around their role in inflammation. Mast cells are recruited within 24 hours of tissue injury, and begin their role by releasing histamine, as well at interleukins such as IL-6 and IL-8, and growth factors. Proteases such as chymase and tryptase are also released from mast cells in the early stages of inflammation, and their role is the breakdown extracellular matrix in preparation for the next stage of tissue healing which is angiogenesis. Mast cells have also been found to have an effect on fibroblast proliferation and angiogenesis. Their production of Vascular Endothelial Growth Factor (VEGF) and TGF-β1 is thought to stimulate fibroblast production and angiogenic growth factors such as fibroblast growth factor-2 have been shown to affect angiogenesis and fibroblast synthesis. Collagen synthesis is another area in which mast cells are known to play a role in. Tryptase that is secreted from mast cells has been proven to increase collagen synthesis.  A considerable amount of research has been conducted on the function of mast cells in tumour growth and maintainance. Activated mast cells can aid tumours by helping maintain a steady blood supply through angiogenesis. Although much research shows that Mast cells promote tumour growth, there is also evidence to suggest mast cells have protective functions when it comes to tumour growth. 
Origin and Migration
Mast cells have a hematopoietic origin from the bone marrow. This was first shown when a patient suffering from acute myeloid leukemia underwent an allogeneic bone marrow transplant and after 198 days post transplant mast cells were found. The researchers used Polymerase Chain Reaction to show that the mast cells being generated by the recipient showed the donor’s genotype, meaning the donors bone marrow was the origin of the Mast Cells 
Committed mast cell precursors are found in the bone marrow. This was shown in murine studies by using mast cell specific antibodies to separate them out immunomagnetically. These precursors had mRNA present for some subunits of FCeR1, the receptor that is characteristic of mast cells, but did not express the receptor on their surface. Hence, they are considered the precursor cells. They also contained mRNA for mast cell specific proteases. .
As stated above research suggests in adults mast cells are produced from haematopoietic origins by a committed precursor in the bone marrow. However in rat embryos, using similar immunological methods, it has been shown that embryonic mast cells mature in the aorta-gonad-meso-nephros region (AGM) which indicates that in human embryos mast cells may develop outside of the bone marrow 
However, mast cell precursors do not differentiate into mast cells in the bone marrow, their precursor cells travel to extramedullary sites with good vasculature such as the connective tissue region of the skin or mucosa of the GIT before differentiation.
The migration of mast cell precursors is tissue specific. It depends on the interaction of the mast cell precursors's integrins binding to the corresponding adhesion molecules in different tissues. For example in the mucosa the adhesin MAdCAM-1 binds to the mast cell precursor's integrins. The precursors also express chemokine receptors that may play a role in migration 
There are both intrinsic and extrinsic factors that stimulate mast cell differentiation from haematopoetic stem cells. The following have been discovered in vitro cell lines from mice that have specific genetic mutations.
|Growth Factor Stem Cell Factor that acts on c-kit receptor, has been shown to stimulate mast cell differentiation in vitro  . There are also growth factors that inhibit mastopoesis such as thrombopoetin, which acts by down-regulating GATA1 ||Transcription factor MitF - Involved in c-kit expression that influences mast cell specific protease expression|
|IL-3 from induced T cells as a result of external stimuli i.e. parasitic infection. It has been shown to be responsible for Mast Cell survival, development and maturation in vitro  but has to be used synergistically with IL-3||Transcription factors GATA2 and GATA1 - Expressed throughout differentiation in vitro. Studies have shown that mutations in GATA1 gene cause defects at all stages of differentiation of mast cell development |
Activation occurs when two or more mast cell FceR1 receptors cross-link after antigen binding. Receptor coupling factors such as LYN and SYK phosphorylate other proteins and downstream effects include :
1) Degranulation: Occurs within 5 minutes due to intracellular Calcium increase as a result of the phosphorylation cascade
2) Eicosanoid production: Occurs between 5 and 30 minutes as a consequence of the MAPK phosphorylation cascade
3) Cytokine production: 30 minutes to several hours after activation, occurs when transcription factors are activated by the MAPK pathway
Mast cells produce an array of bioactive molecules. These are released either by vesicular exocytosis or de novo synthesised and secreted through membrane channels. Moon et al.(2014)  performed a review of the literature and showed the major stored mediators in one table (below).
Stored Mediators: These mediators are stored in vesicles and are usually proteins such as amines, cytokines, or enzymes as shown in the table below . These may or may not be mixed within vesicles i.e. b-hexoaminidase and heparine can both be in one vesicle and secreted together (heterogenic secretion).
Morphometric research shows that the granules that contain mediators result from the fusion of smaller granules (called progranules) that fuse together to form mature granules under regulation of RabGTPases. This has been shown by mutations of RabGTPases gene significantly affecting granule size . The fusion of granules is compartmentalised. Progranules form out of the golgi apparatus and undergo transformation into mature secretory granules that fuse to form "unit granules". These unit granules have larger volumes and may be directly exocytosed.
De Novo Synthesised Mediators: Lipid mediators are de novo synthesised from several locations in the cell. Research suggests eicosanoid lipid mediators can be produced in the membrane of Endoplasmic reticuli, nuclear membranes or lipid bodies . They are made via oxidation reactions of long chain fatty acids . These oxidised fatty acids can then take one of two pathways depending on which enzyme acts on them next. The lipoxygenase pathway produces mostly leukotrienes from arachidonic acid and the cyclooxygenase pathway produces prostaglandins. These molecules are involved in inflammation and do not diffuse freely across the membrane due to their negative charge at physiological pH. As such, they are secreted via transporters .
Electron microscopy ultrastructural research suggests that these granules may be secreted in 2 ways; anaphylactic or via piecemeal degranulation:
|Piecemeal Degranulation||Anaphylactic Degranulation|
|The process where only fragments of vesicles are selectively released without membrane to membrane fusion, the mechanism is rather poorly understood, which involves a pleiomorphic tubular structure that allows a for granule stored proteins to be secreted selectively ||Involves the full exocytosis of granule or vesicle as a result of membrane to membrane fusion|
|Stimulated by Toll-like Receptor activation  and interactions with T regulatory cells  among other situations ||Stimulation is FceR1 cross-linking |
|SNARES protein mediated|
Here is a video depicting degranulation in real time: Video: Mast cell degranulation
Mast cells are sentinel cells that are found distributed within the connective tissue throughout the body and play an important role in both acute and chronic inflammation. Mast cells that are coated with IgE antibodies specific for certain environmental antigens are triggered to release histamine and other cytokines that induce early vascular changes that are hallmarks of acute inflammation.  The immediate responsibility of mast cells is to recognise that infection by a pathogen has occurred, which is achieved by direct recognition of the pathogen by pattern recognition receptors that are activated in response to pathogen-associated molecular patterns (PAMPs).  A study conducted by Supajatura et al. demonstrated that the activation of different toll-like receptors (TLR2 or TLR4) by varying PAMPs resulted in differential activation of mast cells evident in lypopolysaccharide stimulation of TLR4 resulting in cytokine release compared to peptidoglycan stimulation of TLR2 receptors resulting in both degranulation and cytokine production. However, mast cells can also act directly on pathogens through the production of reactive oxygen species and phagocytosis, as demonstrated by the engulfing of Fim-H expressing enterobacteria. 
Mast cells also play a major role in atopic diseases such as asthma, eczema, anaphylaxis and allergic rhinitis. The basis of these allergic diseases is the activation and binding of the high-affinity immunoglobulin E (IgE) receptor FceR1 to initiate receptor clustering and release of mediators, a signalling network dependent on the strength and type of stimulus.  These downstream signal transduction events involve tyrosine phosphorylation which induces the degranulation of mast cells and cytokine and lipid mediator secretion.  Furthermore, similarly to neutrophils, mast cells have been seen to produce extracellular traps through the utilisation of the cathelicidin LL-37, histones and tryptase.  These three products of mast cells form the structural foundation of the extracellular traps, demonstrated to trap the bacteria S. pyogenes when in close proximity in co-culture in vitro. 
Mast Cell Activation Disease
Mast Cell activation disease describes a group of diseases or disorders that are characterised by the accumulation of mast cells in the bone marrow and/or other extracutaneous organs and tissues, and/or the abnormal release of different mast cell mediators.  The three main subsets of MCAD are mast cell activation syndrome (MCAS), systemic mastocytosis (SM); including aggressive systemic mastocytosis and isolated bone marrow mastocytosis; and mast cell leukemia (MCL).  The World Health Organisation (WHO) have defined SM and MCL as truly rare diseases as defined by their criteria, however, it has been suggested that MCAS is more common.  It has also been suggested that pathological mast cells play an important role in the pathogenesis of SM and MCAS, as well as the development of idiopathic anaphylaxis.  
Mutations in enzymes, kinases and receptors that are necessary for mast cell activity regulation and essential in the establishment of a clonal cell population have been implicated as important factors in the pathogenesis of MCAD's, particularly two or more alterations in the tyrosine kinase Kit. The tyrosine kinase Kit mutation occurs at the codon 816 which has been linked with not only good prognosis of SM and conversely advancement of the disease, but also has been identified in healthy subjects, suggesting that there are number of external factors contributing to the pathogenesis.   Further genetic findings suggest that all three subsets of MCAD are clinical manifestations that share one common genetic root associated with mast cell dysfunctions.  
The below table lists the criteria for a diagnosis of Mast Cell Activation Syndrome and Systemic Mastocytosis. It has been adapted from the table published in the review Mast cell activation disease: a concise practical guide for diagnostic workup and therapeutic options  with original reference to the criteria established by WHO. 
|Criteria||Mast Cell Activation Syndrome||Systemic Mastocytosis|
|Major Criteria||1. Presence of pathological mast cells in bone marrow biopsies and/or sections of other extracutaneous organs (multifocal and disseminated).
2. Mast cell mediator release syndrome; characterised by a unique list of clinical complaints e.g. skin lesions, lymphadenopathy and elevated levels of histamine and heparin in the blood.
|Aggregates of >15 mast cells in bone biopsies and/or sections of other extracutaneous organs.|
|Minor Criteria||1. Abnormal morphology of mast cells in bone marrow biopsies and/or sections of other extracutaneous organs. Abnormal morphology includes spindle shaped morphology or expression of CD25 on mast cell surfaces. 
2. Mast cell expression of CD2 and/or CD25 in the bone marrow.
3. Proven increased activity of mast cells detected with genetic changes within mast cells located in the blood, bone marrow and extracutaneous organs.
4. Increase in the content of:
|1. Abnormal morphology of mast cells in bone marrow biopsies and/or sections of other extracutaneous organs. Abnormal morphology includes spindle shaped morphology or expression of CD25 on mast cell surfaces. 
2. Mast cell expression of CD2 and/or CD25 in the bone marrow.
3. Mutation of codon 816 in tyrosine kinase Kit.
4. Serum total tryptase >20 ng/ml.
A patient is initially suspected of having MCAD's based upon a diagnosis of symptoms associated with the over production of mast cell mediators and the identification of skin lesions. However, due to the distribution of mast cells and the heterogeneity of their mediators, clinical symptoms vary greatly between individuals and can affect any organ or tissue.  Some common signs and symptoms are listed below.
There is no curative therapy for patients diagnosed with MCAD's, however effective drug treatments need to be individually tailored, considering the individuals signs, symptoms, complications and drug tolerances. Similarly to the treatment of all disease, avoidance of environmental irritants and reduced exposure to identifiable triggers of mast cell degranulation such as certain medication, animals venoms and animal furs, is an important element of treatment of MCAD's.  Studies had shown that patients diagnosed with SM and treated with kinase inhibitors partially improved clinical symptoms associated with mast cell mediators, as well as normalised mast cell infiltration observed in bone marrow specimens.  
The following table lists some common treatments for MCAD's. It has been adapted from the table published in the review paper Mast cell activation disease: a concise practical guide for diagnostic workup and therapeutic options. 
|Type Of Therapy||Treatment Options|
|Basic Therapy: Oral combination therapy aimed to reduce release of mast cell activity||
|Systemic Therapies: Taken orally as needed to reduce systemic symptoms||
Parathyroid bone disease
An increased number of mast cells in the bone marrow can be linked with parathyroid bone disease, most common of which being chronic hyperparathyroidism (HPT). Those suffering from HPT have a compromised immune function, and mast cells play a major role in innate immunity. Parathyroid hormone (PTH) significantly increases the number of mast cells in those with HPT. There may be up to a 5-fold increase in bone marrow mast cells in those with HPT as compared to the controls.  Elevated levels of PTH increase migration of preoestoblastic fibroblasts to the bone surface, and while these generally differentiate into osteoblasts, the increased PTH levels cause terminal differentiation to be impaired. The accumulation of mast cells on bone surfaces as a response to elevated PTH levels lead to this PTH-induced peritrabcular fibrosis, and cause the excessive recruitment of fibroblasts on bone surfaces.  An increase in kit- ligand expression causes the build up of mast cells, as it is a potent chemotactic factor for these mast cells. Combined with an increased PDFG-A gene expression, these peritrabcular mast cells promote fibrosis, ultimately leading to bone disease. 
Role of mast cells in allergic disease
Mast cells play a key role in allergic diseases, including asthma, eczema and allergic rhinitis. Allergic reactions are significantly characterised by an early and late phase response, subsequently followed by the initiation of inflammation. Mast cells play a key role in both of these phases, with the early response being initiated when mast cells release preformed mediators from their granules. These include histamine, chymase and trypase. Furthermore, mast cells also produce prostaglandin D, leukotriene C as well as a range of other mediators that cause the symptoms of bronchoconstriction, mucous secretion and edema formation in allergic reactions.  These also typically lead to early vascular changes that can cause acute inflammation 
Mast cells are also significantly involved in generating the late phase response of allergic reactions, and this is due to the mast cell-dependent secretion of proinflammatory cytokines and chemokines.  Here, they act in immune-regulatory cytokine cascades, by both initiating an amplifying the cytokine responses.  In particular, they can initiate and amplify specific cytokines such as TNF, as well as other chemokines, and can play a major role in resisting some allergic infections. 
Allergic Rhinitis is one of the most prevalent atopic diseases affecting approximately 400 million people world wide and is associated with reduced productivity, reduced quality of life and a lower learning performance in schools.   Allergic Rhinitis is a disease that affects the mucosal membrane of the nose and is mediated by Immunoglobulin E (IgE), a product of mast cells. The complex interplay between immune system and an allergen results in the release chemokines, cytokines and mediators (such as histamine release by mast cells) leads to clinical manifestations including nasal blockage, sneezing and allergic conjunctivitis.
Similarly to all allergic diseases, Allergic Rhinitis can be divided into an immediate phase and a late phase. The immediate phase, occurring within minutes of exposure to the allergen, is characterised by mast cell degranulation, release of pre-formed and newly formed mediators (such as histamine) stimulating the nerve endings of the trigeminal nerve (CN5) and inducing sneezing. The late phase response, occurring 4-6 hours after antigen stimulation, is driven by mast cell release of chemokines including IL-4 and IL-13, which upregulates the expression of adhesion molecules on endothelial cells resulting in an increased infiltration of immune cells (eosinophils, basophils, etc.) into the nasal mucosa.  Mast cells have been found to further contribute to the late phase through the histamine-tyrptase induced upregulation of granulocytemacrophage colony stimulating factor and the chemokine RANTES in nasal epithelial cells. 
The following table discusses some potential treatments for Allergic Rhinitis. It has been adapted from a flow chart/figure presented in the review article Overview of the Treatment of Allergic Rhinitis and Nonallergic Rhinopathy. 
Asthma is an allergic disease significantly characterised by variable airflow obstruction as well as airway hyper responsiveness.  It causes repeated episodes of wheezing, tightness in chest, breathlessness, as well as coughing during night or early morning. There has been a significant increase of asthma incidence in the West over the past four decades. 
Asthma occurs through the accumulation of eosinophils, CD4+ lymphocytes in the submucosa, as well as mucous gland hyperplasia and mast cell degranulation.. IL-18, a proinflammatory cytokine promotes the production of type 2 helper T cells, which allows for most of the features of the disease, including IL-13 promoting IgE production.  This coats the submucosal mast cells, which when exposed to the allergen, release the granule contents.  This subsequently leads to the induction of a two wave reaction – the early phase and late phase reaction, in which the early phase is characterised by bronchoconstriction and increased mucus production, and the late stage responses involves inflammation through activation of eosinophils, neutrophils and T cells. 
Mast cells are found to be localised in the airway smooth muscle, and the subsequent interaction between mast cells and the smooth muscle cells is an important factor in the occurrence of asthma, due to the fact that the smooth muscle provides the appropriate microenvironment for the differentiation, activation and survival of mast cells. 
As shown in the adjacent image, the mechanisms leading to an infiltration of the airway smooth muscle (ASM) layer by mast cells in asthma firstly involves mast cell chemotaxis towards the ASM bundle. This is subsequntly followed by direct mast cell-ASM call adhesion. The mast cell- extracellular matrix adheres to the ASM cell, leading to mast cell activation. Once this occurs, mast cells release mediators which in turn activate ASM cells such as TNF-α and tryptase. ASM cells then produce and secrete chemotactic factors for mast cells, thus leading to an auto-activation loop. Under the influence of Th1, Th2 and/ or pro-inflammatory cytokine stimulation, ASM cells also secrete a range of mast cell chemotactic factors. 
Inhaled corticosterioids (ICS) have become the primary controller therapy for asthma. The standard treatment for chronic or persistent asthma is typically determined by symptom control, and younger people suffering from asthma will generally have a positive response to ICS.  In addition, there are several identified phenotypes of asthma that can respond to therapies direct toward TH2 immune pathways as treatment methods.  While milder stages of asthma can be treated, sever asthma, accounting for 5-10% of overall asthma patients, remains difficult to treat due to a lack of complete understanding of its physiology and pathology. Currently, therapies do not completely control the symptoms of severe asthma and even intensive treatment has little effect. 
Atopic dermatitis, or eczema, is a chronic inflammatory skin disease  characterised by severe puritus and high Immunoglobulin E (IgE) responsiveness.   While the aetiology of the disease is not fully understood, it is caused due to an interaction between environmental and genetic factors, particularly involving high levels of IgE.  IgE is produced in response to common antigens, and is bound to the surface of mast cells,  which infiltrate the skin lesions of the disease. . Thus, an accumulation of mast cells is required for maximum skin inflammation during eczema.  The IgE proceeds to bind to FceRI, and consequently has a positive effect on mast cell survival and activation.
In eczema, the impaired skin barriers allow allergens easy access into the dermal and epidermal layers. The allergens are taken up by Langerhans cells, and these cells mature in order to present the allergens to helper T cells in the lymph nodes. Activated Th2 cells migrate once again to the skin sites that are re-exposed to the allergens, and subsequently recruit mast cells which cause the characteristic tissue damage and irritation of eczema. 
Mast cell chymase (MCC) is also involved in this condition, and is a serine protease which accumulates in the dermis of the skin. An increase in MCC promotes skin inflammation and eczema. The polymorphism of MCC is significantly associated with the occurrence of eczema, and different variations of this MCC is one source of genetic risk for the condition. 
The main course of treatment of eczema is the use of corticosteroids and calcineurin inhibitors, and these are used regardless of clinical severity. In patients with cases of severe recalcitrant eczema, cyclosporine (CS) is commonly used now. CS selectively inhibits T-cell activation, and inhibits keratinocyte hyper proliferation and the release of histamine from mast cells, hence allowing it to be an effective treatment option for this disease.  Long term treatment of the disease further involves the anti-inflammatory therapy through topical glucocorticosterioids applied accordingly as dictated by the degree of severity of the skin lesions. Furthermore, newer, proactive approaches to treating eczema and atopic diseases in general involve intensive use of this anti-inflammatory therapy until the lesions have cleared, followed by low dose intermittent application of anti-inflammatory agents to the affect skin areas to prevent the condition occurring once again. 
Allergic Rhinitis: Commonly known as hay fever
Angiogenesis: The development of new blood cells
Atopic disease: A clinical condition caused by an allergy
Atopy: Predisposition to developing IgE associated allergic diseases
Bronchoconstriction: Constriction of the airways in the lung due to the tightening of surrounding smooth muscle, leading to coughing and shortness of breath
Chemokines: Small family of cytokines that are able to induce chemotaxis in nearby cells
Cytokines: Small secreted proteins released by cells have a specific effect on the interactions and communications between cells 
FceR1: High affinity Immunoglobulin E receptor
Histamine: A chemical released by mast cells when tissue is injured or in allergic and inflammatory reactions, causing dilation of small blood vessels and smooth muscle contraction
Hypersensitivity: A series of damaging reactions produced by the normal immune system
Hyper-responsiveness: Having an abnormal degree of responsiveness to the original trigger
Idiopathic: A disease with an unknown aetiology
Interleukin: Any of a class of glycoproteins produced by leucocytes for regulating immune responses
Immunoglobulin E (IgE): Antibodies produced by the body's immune system
Kinase: A transferase that catalyzes the phosphorylation of a substrate by ATP
Lymphocytes: Small white blood cells that are present especially in the lymphatic system
Osteoblast: Form closely packed sheets on the surface of the bone, from which cellular processes extent through developing bone
Pathogen-associated molecular patterns (PAMPS): Molecules associated with pathogens that are recognised by immune cells
Pattern recognition receptors (PRR): A class of innate immune response-expressed proteins
Phosphorylation: The chemical or enzymic introduction into a compound of a phosphoryl group through the action of phosphorylase or a kinase
Protease: An enzyme which breaks down proteins and peptides.
Proteoglycan: A macromolecule composed of a polysaccharide joined to a polypeptide and forming the ground substance of connective tissue
Pruritic: Relating to an itching or scratching sensation
Reactive Oxygen Species: Chemically reactive molecules containing oxygen that are formed as a natural byproduct of oxygen metabolism
TGF-β1: Transforming Growth Factor Beta 1
TNF: Tumour necrosis factor
Toluidine blue: A basic thiazine dye that is related to methylene blue and is used as a biological stain
Vascular Endothelial Growth Factor (VEGF): A signalling protein that promotes the growth of new blood vessels
- Ehrlich, P. (1878) Beitra¨ge zur Theorie und Praxis der Histologischen Fa¨rbung. Thesis, Leipzig University.
- Unna PG. Die spezifische färbung der mastzellenkörnung. Monatsh Prakt Dermatol. 1894;19:367–368.
- Jolly, M.J. (1900) Clasmatocytes et mastzellen. Compte Rendus Socie´te´ de Biologie (Paris), 52, 437–455.
- Jorpes JE. The site of formation of heparin. In: Jorpes JE, editor. Heparin: Its Chemistry, Physiology, and Application in Medicine. London, United Kingdom: Humphrey Milford (Oxford Univ Press); 1939. pp. 30–39.
- <pubmed>18258601 </pubmed>
- Netters Essential Histology. Ovalle, William k, Nahirney, Patrick C. Elsevier Health Sciences 2007
- <pubmed> PMC3318920</pubmed>
- <pubmed> PMC3343118</pubmed>
- <pubmed> 11490012</pubmed>
- <pubmed> 17498052</pubmed>
- <pubmed> PMC3318920</pubmed>
- Widmaier, Eric P et al. Vander, Sherman, & Luciano's Human Physiology. Boston: McGraw-Hill Higher Education, 2004. Print.
- Robins Basic Pathology Kumar, Vanay; Abbas, Abul K.; Aster, Jon C., Philadelphia: Elsevier Saunders., 2013
- Robins Basic Pathology Kumar, Vanay; Abbas, Abul K.; Aster, Jon C., Philadelphia: Elsevier Saunders., 2013
- <pubmed> 7535336</pubmed>
- Robins Basic Pathology Kumar, Vanay; Abbas, Abul K.; Aster, Jon C., Philadelphia: Elsevier Saunders., 2013
- <pubmed>18445188 </pubmed>