Allergies are an exaggerated immune system response to substances typically harmless to most individuals, such as pollen, pet dander, or certain foods. The body mistakenly identifies these as threats, launching a defensive reaction. This raises questions about when humans first encountered such reactions and why they appear increasingly common today.
Historical Insights
Observations resembling allergic reactions date back millennia. Pharaoh Menes of ancient Egypt (c. 3000 BC) is an early documented case, believed to have died from anaphylactic shock due to a wasp sting. Ancient Egyptian medical texts, like the Papyrus Ebers (19th century BCE), describe symptoms akin to eczema, asthma, and colds, noting reactions to strong flower odors. These accounts suggest a long human susceptibility to environmental triggers.
Hippocrates (5th century BC) used “idiosyncrasy” to describe conditions similar to eczema and asthma. He observed respiratory problems among blacksmiths and stonemasons, indicating early recognition of occupational asthma. The poet Lucretius also noted that “food for most” could be “poison for some,” hinting at food sensitivities.
Later, Persian physician Rhazes (9th or 10th century AD) detailed seasonal rhinitis symptoms, noting their worsening in spring. In 1565, Leonardo Botallo described “rose catarrh”—symptoms including headache, sneezing, and an itchy nose in the presence of roses. “Summer asthma” emerged around 1600, acknowledging seasonal sensitivities. John Bostock provided a detailed description of hay fever in 1819, and Charles Harrison Blackley identified pollen as its cause in 1859. The term “allergy” was formally introduced by Clemens von Pirquet in 1906.
Evolutionary Perspectives
The human immune system’s capacity for allergic responses is rooted in our evolutionary past. The “hygiene hypothesis,” proposed by David Strachan in 1989, suggests that reduced early childhood exposure to infections, often seen in cleaner environments, can lead to a higher incidence of allergic conditions like hay fever and eczema. The immune system requires “training” through encounters with diverse microbes to properly differentiate between harmless and harmful substances.
Graham Rook refined this idea in 2003 with the “old friends hypothesis.” This theory emphasizes that crucial microbial exposures are not merely common childhood infections but microorganisms with which humans co-evolved. These “old friends” include gut microbiota, certain parasites, and environmental microbes consistently present in ancestral environments.
These co-evolved microbes help tune the immune system and promote regulatory T cells, which are important for maintaining immune tolerance. A lack of sufficient exposure to these microorganisms can result in immune dysregulation, causing the immune system to overreact to benign environmental substances. This perspective helps explain the increase in inflammatory disorders observed in modern societies.
Contemporary Factors
The rapid increase in allergy prevalence in modern societies is a notable phenomenon, with rates of many allergic diseases continuing to rise. This surge is attributed to factors linked to contemporary lifestyles and environments. While the underlying capacity for allergies is ancient, their widespread manifestation is more recent.
A primary factor is the continued reduction in microbial diversity in our environments, a modern interpretation of the hygiene hypothesis. Changes in birth practices, such as increased Cesarean sections, can alter an infant’s initial gut microbiota exposure. Early antibiotic use and reduced breastfeeding can further impact the developing gut microbiome, leading to an imbalance in bacterial composition.
Dietary shifts also play a role. Western-style diets, characterized by low fiber and high processed foods, affect the gut microbiota. These diets often contain increased levels of polyunsaturated and omega-6 fatty acids, which can promote inflammation. Historical changes in infant feeding guidelines, such as delaying the introduction of certain allergenic foods, may have contributed to the rise in food allergies.
Environmental pollutants are another significant contributor. Air pollution, including diesel exhaust particles, tobacco smoke, ground-level ozone, and nitrogen oxide, can worsen existing allergies and make allergens more potent. These pollutants can attach to pollen and other allergens, potentially increasing their ability to trigger allergic reactions.
Indoor environments, where people spend a significant amount of time, also contribute to allergen exposure. Modern, well-insulated homes with reduced ventilation can trap indoor pollutants like dust mites, mold spores, pet dander, and VOCs. Poor ventilation and higher indoor humidity create ideal conditions for the proliferation of dust mites and mold, exacerbating allergic symptoms.
Alterations in the gut microbiota are increasingly recognized as a central mechanism in allergy development. An imbalanced gut bacterial community, or dysbiosis, can impair the immune system’s ability to develop tolerance to harmless substances. Beneficial bacteria, such as Clostridia, Lactobacillus, and Bifidobacteria species, produce short-chain fatty acids (SCFAs) that regulate immune pathways and reduce inflammation. A lack of these beneficial microbes can compromise this immune regulation.
The “dual allergen exposure hypothesis” suggests the route of allergen exposure influences immune response. Exposure through compromised skin, as seen in conditions like eczema, can lead to sensitization and the development of allergies. Conversely, early and consistent oral exposure to potential allergens can promote immune tolerance. This understanding highlights how modern factors, by altering microbial exposure and promoting skin barrier issues, contribute to the increasing prevalence of allergic diseases.