According to the World Health Organization (WHO) and the World Allergy Organization (WAO), allergies or immediate-type hypersensitivity reactions, affect approximately 20-30% of the world population. Allergies are the most common chronic diseases in adults and children in developed countries, accounting for high levels of morbidity and social burden, as determined by various direct and indirect parameters. These parameters include self-reported symptoms, medication use, hospitalizations, emergency room admissions, and missed work or school days.
The prevalence and incidence of allergies has increased during the past decades and continue to increase in developed countries. Several hypotheses, including the “Hygiene Hypothesis”, have been proposed to explain this phenomenon. This hypothesis postulates that the decreasing exposure to microbial products, resulting from civilization, plays a role in switching Th1 to the Th2 immune response. Th1 responses protect from developing allergies while Th2 responses promote them.
Many individuals in the world live in urban areas, which have grown during the past years. Developing countries are evolving, an over 90% of urban growth is occurring in those nations, adding an estimated 70 million new residents to urban areas each year. Therefore, an increase in the prevalence and incidence of allergies in developing countries should be expected over time.
Multiple factors responsible for allergies have been identified. The factors responsible for allergies are complex and result from a combination of both genetic and environmental components.
Allergies cluster in families. The risk of developing allergies in children is estimated to be around 30% if one parent has allergies and 70% if both parents suffer from them. A large number of genes have been associated with allergic asthma and particular allergic phenotypes.
The most important environmental factor responsible for allergies is exposure to allergens. Other environmental factors that can promote allergic sensitization and trigger symptoms, particularly asthma, include exposure to air pollutants.
Infants with particular genetic phenotypes predisposing to allergies initially develop food allergy, resulting in atopic dermatitis, which generally resolves with increasing age. It has been reported that approximately 40%-70% of those children are symptom-free when they reach an age of approximately 7 years.
In many cases, children with food allergies subsequently develop allergic rhinitis, resulting from exposure to aeroallergens. Allergic rhinitis often evolves to allergic asthma. This progression in the manifestation of allergic symptoms is known as “the allergic March”
Due to the complex interaction among the various factors responsible for allergies, a number of long-term birth cohort studies have been and are being conducted to ascertain the relevance of the combined factors responsible for allergic sensitization and its progression from childhood to young adulthood. These studies include the International Study of Asthma and Allergies in Childhood (ISAAC), Mechanisms of the Development of Allergy (MeDALL), Developing a Child Cohort Research Strategy for Europe (CHICOS), Environmental Health Risks in European Birth Cohorts (ENRIECO), and Global Allergy and European Network (GA2LEN). These studies will identify the factors responsible for allergies.