**3. Signs and symptoms**

Allergens are proteinaceous molecules that can be found in diverse substances in various forms. These can be inhaled, ingested and can also be exposed through contact to skin. Many allergens present in dust and pollens are airborne particles. These can be easily exposed through air and can induce symptoms in areas that contact


#### **Table 2.**

*Common symptoms of allergic reactions [13].*

with allergen such as eyes, nose and lungs. Most common symptoms like hay fever also known as allergic rhinatisis cause runny nose, mucosal irritation and sneezing [13]. Some can also swollen eyes with itching and redness. Inhaled allergic particles can get into lungs and lead to bronchial hyper responsiveness. Particulate allergens inhaled through air can enter the lungs and cause asthmatic symptoms. Narrowing of the airways induce sneezing, coughing and through bronchoconstriction. The increased production of mucus thickens the airways and restricts the airflow to lungs that cause shortness of breath (dyspnea, bronchial hyper responsiveness and wheezing. Apart from these, the allergic reaction can be encounter through contact of allergens, ingestion through food and medications, insect bites and drug administration [13, 14]. Symptoms of contact and food allergy include itchy and swelling of the skin found during hives, gastrointestinal upset, edema, vomiting and diarrhea. Food allergies rarely cause respiratory (asthmatic) reactions, or rhinitis (**Table 2**).

Insect bites, drugs, medications and contact to insect stings with venom produce systemic allergic response affecting multiple organs. The exaggerated hyper immune response which is acute, life threatening and serious is called anaphylaxis and if not attended may induce death. The allergens effect multi organ system including digestive, respiratory, circulatory and cardiac system. Based on the severity and rate of sensitization the allergens can cause cutaneous reactions, edema, hypotension, bronchoconstriction, coma and sometime death [13]. Many allergenic substances such as latex can induce contact dermatitis and angioedema through skin rashes and irritations. The nature and source of allergens are diverse and they cause both cutaneous and systemic symptoms which range from very mild to severe depending on route of exposure and sensitization mechanism.

### **4. Causes**

Causative agents for allergy or hypersensitivity reactions were allergens which present in many diverse sources in the environment. These allergy inducing factors have been placed in two categories (i) host factors and (ii) environmental factors [15]. The human host for the allergic reactions has different immune sensitivity due to various host factors that include gender, race, heredity and age. The genetic makeup and hereditary predisposition forms the basis for the increased incidence of allergic at certain population. However, there is insufficient evidence to explain the increase in allergic disorders with genetic factors alone. The change in food habits,

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

living style and environmental pollutions and microbial exposure make huge contribution in allergic incidence. There were major environmental factors that alter the immune sensitization to induce atopy. To mention, the exposed inhalant and ingested allergen levels, exposure to infection diseases during early childhood and dietary changes. The alteration in environment certainly induces immune modulation that favors the development of allergic disease in susceptible population. The major class of allergens that predominantly cause allergic reactions belong to one of the following categories.
