**6.1 Modes of activation of mast cells/basophils**

Cross linking of adjacent IgE by an allergen induces activation and degranulation of granulocytes (mast cells and basophils). The binding of IgE to cell surface is possible due to the molecule that has ability to bind IgE with high affinity and keeps it attached to cells surface. The Fc epsilon R1 receptor has strong affinity to Fc portion *Immunological Basis for the Development of Allergic Diseases-Prevalence, Diagnosis… DOI: http://dx.doi.org/10.5772/intechopen.95804*

#### **Figure 3.**

*Biochemical events of mast cell or basophil activation. There is a sequence of biochemical events following the bridging of cell bound IgE by an antigen. The climax of the whole process is the non-cytotoxic secretion of various chemical mediators such as histamine and other pharmacological substances.*

of IgE which is made up of epsilon class of constant region. The allergen interacts with specific epitope to fragment of antigen binding (Fab) portion of the IgE. The allergen should be multivalent and with higher size so that it cross-link the two adjacent IgE to induce activation of mast cells for degranulation [30] to induce allergic reaction.

#### **6.2 Cells and components of immune system involved in allergic reactions**

The components of immune system are responsible for hyper immune response against allergen. Many cells and antibodies, cytokines are involved in various immune function that results in allergy or atopic reaction. The granulocytes like mast cells, basophils and eosinophils, lymphocytes such as Th2 cells and B cells play a prime role in development of allergic reactions. Inter molecular complex formed between allergen, IgE and FcεRI on the surface of mast cell or basophils are essential for activation of degranulation process to release mediators. That forms the basis for the immune activation to induce allergic diseases.

**Atopic allergens:** The allergens responsible for atopic diseases are derived principally from natural and airborne organic particles, especially plant pollens, fungal spores, and animal or insect debris, and to lesser extent from ingested food [31]. The ability of different pollens, molds, or foods to sensitize for IgE allergy varies, so that some of these environmental allergens are intrinsically more sensitizing than others, irrespective of the amount of exposure. Some lectins have been identified as allergens peanut agglutinin [32], soybean agglutinin [33] and wheat germ agglutinin [34] and are in general recognized as minor allergens in comparison with other common major allergens.

**Mast cells and basophils:** Mast cells are mononuclear cells with densely stained metachromatic granules while basophils are polymorphonuclear and are smaller in size (**Table 3**), approximately 5–7 microns versus 10–15 microns [35].

All the circulatory and connective tissues of the human system are susceptible for allergic response. It is due to the distribution of mast cells and basophils almost in all parts of the body. Mast cells are distributed essentially in all connective body parts and are often find adjacent to epidermal and microvasculature. The development, maturation and differentiation of mast cells influenced by cytokines and cellular growth factors like stem cell factor (c-kit ligand). Whereas basophils found in circulatory system and through hematopoietic cell linage precursor cells which are differentiated and matured from myeloid progenitor cells into specialized granulocytes along with eosinophils [36]. The basophils are circulatory and move through the blood and represent around 1% of the leukocytes. Mast cells are static and are found adhered to connective tissues across the body. These two cells contain preformed allergic pro-inflammatory mediators in the cellular granules which on degranulation cause allergic inflammations.

**FcεRI and immunoglobulin E (IgE):** The mast cells and basophils have high affinity receptors which has specificity to bind Fc portion of IgE antibody called Fc epsilon R1. This consist of four subunits (αβγ2) which represent one extracellular alpha (α) domain which is need for IgE binding. The beta (β) subunit is a transmembrane domain which spans the plasma membrane and the gamma (γ) subunits present as intrinsic membrane protein and are responsible for signal transaction [37]. The FcεRI binds IgE with high affinity (109 to 1010/mole) and that is important phenomena for allergic response and development of allergy.

The Immunoglobulin (IgE) is referred as regenic antibody and it play a important role in allergic hypersensitivity reaction. It is a glycoprotein and belongs to one of the class of antibody with molecular weight of 190 kD and has 12% carbohydrate by weight which is present in the heavy chain at Fc portion. The serum concentration of IgE ranges from ng/mL to μg/mL with an atopic serum half-life of 2–3 days [38]. IgE recognizes mast cells and basophils through FcεR1 receptor and gets inactivated by heating at 56°C for at least 30 min. The cytokines IL-4, IL-13 and IL-10 induce synthesis of IgE by plasma cells. The detection allergen-specific IgE antibodies in the individual sera are considered as prominent diagnostic parameter and represent the allergic sensitivity.

**Th2 or CD4+ cells:** T helper cells are the immune responsive cells that have special interest in humoral immunity through induction of antibody production. The atopic individuals have high circulating allergen specific IgE antibodies. TH cells are circulatory lymphocytes which are characterized as CD4+ cells. There are two subsets of T-helper cells based on the antigen recognition and cytokine secretion. The Th1 cytokines direct the B cell to induce IgM and IgG. In contrast the Th2 type of response produce cytokines IL-4, IL-5, IL-10 and IL-13 [39, 40] these direct the B lymphocytes (B-cells) to produce allergic immunoglobulin IgE. This differentiates the function of Th2 from Th1-type cytokine (IFNγ and IL-2) response. The immunopathological hallmark of allergic disease is the infiltration of affected tissues by cells with a Th2 type cytokine profile [41, 42] that increase IgE production and allergic reactions.

**Mediators released by mast cells and basophils:** The pathophysiology of allergic reaction is exhibited with the inflammatory symptoms which are initiated by various allergic mediators released through the degranulation of mast cells and basophils. These cells synthesize and prestore granular mediators and instantly generate the lipid mediators [41]. The granular preformed mediators are rapidly released following activation; these represents; histamine, tryptase, serotonin, and other inflammatory cytokines. The others are synthesized de nova following mast cell activation and are release slowly. These include prostaglandin and leukotrienes that are metabolites of membrane lipids [43]. The complete list of mediators from mast cell and basophils are quite extensive and are undoubtedly account for multiple possible pathological consequences of allergic reactions.

### **7.** *In vivo* **and** *in vitro* **diagnosis of allergic reactions**

Allergic disease needs diagnosis and prognosis for constant monitoring and treatment of symptoms. When allergic reaction is suspected in an individual based

#### *Immunological Basis for the Development of Allergic Diseases-Prevalence, Diagnosis… DOI: http://dx.doi.org/10.5772/intechopen.95804*

on the symptoms, the systemic diagnosis is essential for detection of causative allergen. A detailed case history of exposure and duration for appearance of symptoms with possible repetitive incidence will provide an idea of the type of allergy. The allergy diagnosis varies form case to case and it needs extensive inspection to identify the possible causative agent; the allergen. Based on the case history and information some can be identified rapidly however, the complex, obscure cases need repetitive interceptions to find the allergen. History, physical examination, onset of disease, duration of symptoms, time for resolving symptoms and kind and organ affected are required for initial assessment. This is followed by specific laboratory test which are required for the diagnosis and conformation of the onset of allergic reaction like eosinophils counts, total serum IgE levels, serum histamine levels and related medical examinations (**Figure 4**). It is important to correlate the detailed case history with the laboratory tests and that provide evidence of allergy [44].

Allergic disease is often episodic and that depends on the exposure to allergen to which the individual is sensitive. The case specific objective signs of the allergic symptoms can only be identified during the allergic incidence with proper physical examination. The observed symptoms have to correlate with subjective signs provided in the case history for the proper identification of allergic disease. Allergy diagnosis requires thorough examination to rule out other illness of the subject. A variety of *in vitro* and in vivo laboratory tests are available to supplement the history and physical examinations. There are qualitative and quantitative tests that predict the allergic reaction using sampling fluids and immune cellular responses through immunochemical techniques.

Many of the allergic symptoms shares common pathological behaviors with other illness and that need confirmative cross examination before planning

#### **Figure 4.**

*Schematic diagram showing the identification and assessment of atopic status using clinical history, physical examination and laboratory tests.*

treatment regime. For instance, the viral flu induce rhinitis and nasal conjunction which also the same with pollen or dust allergy. The food toxicity or certain diet can cause gastrointestinal upset which is quite same as that of food allergy. The common cold or viral flu airway infections induce bronchoconstriction that results in wheezing which exactly mimics the symptoms of allergic asthma. Henceforth, careful diagnosis is a prerequisite for the identification and treatment of allergic disorders.

## **7.1 Skin testing**

Skin testing is the in vivo mimicking of allergic reaction that demonstrates the allergic sensitivity to specific allergen. The skin testing predict and confirm the presence of allergen specific IgE antibodies in the individual. These were most preferred over the blood testing during allergy diagnosis. Skin testing is also known as prick test and puncture testing. The most two types of allergic tests, which are commonly in use at clinical level for diagnosis are skin prick test (SPT) and prick by prick test (PPT). In the earlier one the suspected allergen sample was placed and was pricked with small needle and allowed for erythoma formation for 30 mins [45]. In later the sample was pricked initially with the needle and then the same was pricked to skin and the pricked area was observed for the development of reaction. The histamine was used as positive control and PBs as negative. The wheel and flare diameter was measured and was compared for the prediction of positive allergic reaction. Some time a similar intradermal test on the skin can also be used for assessment of allergic reaction to certain medication and drugs. The skin testing is widely used in allergic clinic with standard available panel of allergen samples to identify causative allergen and provide proper treatment for allergic symptoms.

#### **7.2 Blood testing**

Blood is the primary biological sample for diagnosis of illness in clinics. The blood sample contains various immune components that are related to allergic reactions [46]. Various blood allergy testing parameters and methods are available which can detect and diagnose allergy and identify allergens. The most often used are serum total IgE level; that estimate the IgE content in the subject serum per mL. The other is allergen specific IgE level which predicts the confirmative diagnosis of elicitor. Both are measured through radiometric (RAST) or colorimetric (ELISA) immune assays.

#### **7.3 Other methods of testing**

**Allergen challenge testing:** During allergen challenge test, the subject was monitored and the whole procedure was done in the presence of a expert clinician. In this, a small amount of suspected allergen was introduced to subject through oral or other routes and appearance of allergic reactions were monitored. This test provides confirmative evidence and identifies the causative allergen.

**Elimination/Challenge tests:** In this procedure, subject was instructed to avid coming in contact with allergen prior to test. During asymptomatic time, few suspected allergens were added with food or medicines and were given to subject and the appearance of allergic symptoms was recorded. Based on this a true allergen can be identified for planning treatment.

**Patch testing:** Patch testing is much in practice for identifying the contact dermatisis or delayed type of allergic reactions. In this case an allergen is placed on the patch and that is stick to the back of the subject. The symptoms will be observed after 24 hrs for the appearance of symptoms.

*Immunological Basis for the Development of Allergic Diseases-Prevalence, Diagnosis… DOI: http://dx.doi.org/10.5772/intechopen.95804*

**Unreliable tests:** There are some allergic tests which are not considered for practice by International allergy council and those does not provide proper scientific evidence to identify allergy or allergens. Some of them are cytotoxicity testing, provocative tests, subcutaneous or sublingual testing. In future with substantiate research and technical improvement can be used for diagnosis of some of the allergic diseases.
