Why do we have allergies?

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Understanding the Origins and Increase of Allergies

Introduction to Allergies: Immune System and Environmental Factors

Allergies are immune system reactions to typically harmless substances, known as allergens. These reactions are primarily mediated by Immunoglobulin E (IgE) and T helper (Th) cells, which trigger inflammation and other symptoms in various organs such as the skin, gastrointestinal tract, and respiratory system . The prevalence of allergic diseases, including asthma, atopic dermatitis, and food allergies, has significantly increased over the past few decades, particularly in developed countries .

Genetic and Environmental Influences on Allergies

Genetic Predisposition and Immune Response

Genetic factors play a crucial role in the development of allergies. Individuals with a hereditary predisposition to produce IgE antibodies against common environmental allergens are more likely to develop allergic diseases. This genetic susceptibility is often coupled with a Th2-dominated immune response, which is pivotal in the pathogenesis of allergic reactions.

Environmental Changes and Lifestyle Factors

The dramatic rise in allergy prevalence is largely attributed to environmental changes and lifestyle factors. Increased exposure to sensitizing allergens, reduced immune system stimulation during critical developmental periods, and changes in diet and microbiome composition are significant contributors . Urbanization, climate change, and loss of biodiversity have also been identified as key factors affecting the sources and concentrations of aeroallergens and pollutants, further exacerbating allergic conditions.

The Role of the Microbiome and Epithelial Barrier

Microbiome Alterations

Disturbances in the gut and organ-specific microbiomes are critical in the development of allergies. The composition and function of the commensal microbiota, particularly early in life, influence immune tolerance and the body's response to allergens . Modern lifestyle changes, such as increased hygiene and antibiotic use, have altered the microbiome, potentially leading to a higher incidence of allergic diseases.

Epithelial Barrier Dysfunction

Damage to epithelial surfaces in the lungs, gut, and skin is a fundamental factor in the onset of allergic diseases. The epithelium acts as a sensor of environmental stimuli, and its dysfunction can lead to a type-2 immune response, driving atopic conditions. Factors such as air pollution, food allergens, and chemicals in modern detergents contribute to epithelial barrier damage, promoting allergic reactions.

Mechanisms and Pathophysiology of Allergic Reactions

Allergen-Specific Immune Responses

Allergic reactions are initiated by allergen-specific Th cells, which activate a complex immune response involving the release of potent mediators and recruitment of inflammatory cells. This process leads to the clinical symptoms of allergic diseases, such as inflammation and tissue damage. Understanding these mechanisms has opened new avenues for therapeutic interventions aimed at controlling symptoms and suppressing inflammation.

The Exposome Concept

The concept of the exposome, which encompasses the totality of environmental exposures from preconception onward, is crucial in understanding the development and exacerbation of allergic diseases. Indoor and outdoor aeroallergens, air pollutants, and other environmental factors play a significant role in the inflammatory response to allergens. This holistic approach highlights the need for comprehensive strategies to mitigate environmental impacts on allergy development.

Conclusion

The increasing prevalence of allergies is a complex interplay of genetic predisposition, environmental changes, and lifestyle factors. Understanding the mechanisms underlying allergic reactions, the role of the microbiome, and the impact of environmental exposures is essential for developing effective prevention and treatment strategies. As research continues to unravel the intricacies of allergic diseases, a systems approach that considers both genetic and environmental factors will be crucial in addressing this growing public health concern.