**2. Epidemiology**

Prevalence of allergy or allergic disease fundamentally depends on various factors that govern the susceptibility of population to develop atopic condition. Predominantly genetic and environmental predisposition frames the basis for occurrence of allergy in an individual. Globally 8–10% of the population suffers from one or the other type of allergic disease which range from mild rhinatisis to sever asthma or anaphylaxis. At present a steady increase in the atopy was observed due to change in lifestyle, food habits and environment (**Table 1**). Several hypothesis and study provide evidence of genetic change in the population due to increased immune sensitivity and reduced antigen tolerance. Some report had identified an increase in allergic prevalence due to increase in perennial allergens exposure that happened by housing changes, increase in indoor environment with reduced ventilation and change in hygiene approach that decreased activation of immune regulatory control. The change in dietary habit, increased obesity, reduced physical exercise adds to hyper immune sensitization that increase atopy [11]. The reduced exposure to wild and native environmental antigens and high hygienic living standards expose fewer infections. It is reported that reduced infection at early

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


#### **Table 1.**

*Allergic conditions: Statistics and epidemiology [11, 12].*

childhood age direct and polarize the developing immune system from Th1 type Th2, that makes the normal harmless antigen to a dangerous hypersensitive allergen that allow an increase in allergic disease.

Decreased rate of exposure to infection is not only increase immune sensitivity it also polarize immune response towards atopic mechanism. The hygienic hypothesis alone was unable to explain the increased prevalence of allergic disease. The recent evidences provide, importance of gastrointestinal microbial environment in development of atopy. Gut health, food and fecal-oral pathogens substantiate greater role to decide the risk for development of atopy. In some studies it was observed that an increased parasitic infection has been shown to associate with decreased prevalence of asthma [12]. This indicates the infection can exert the effect on Th1/Th2 regulation and it speculate that the dominance in Th1 response decrease atopy and Th2 link to IgE induction and elevates allergic diseases.
