Hygiene Hypothesis

The Hygiene Hypothesis is a scientific model that links the increasing prevalence of allergic diseases such as asthma and allergic rhinitis, as well as autoimmune diseases such as type 1 diabetes, multiple sclerosis (MS), and inflammatory bowel diseases (IBD), in industrialized societies since the second half of the twentieth century to reduced exposure to microbial agents during early childhood.

First formulated in 1989 by David Strachan, who observed an inverse relationship between the number of children in a family and the incidence of hay fever, the hypothesis was initially proposed only for allergic diseases but was later expanded to include autoimmune conditions. Its central premise is that improvements in hygiene conditions, changes in dietary habits, and urbanization associated with modern lifestyles reduce microbial stimuli necessary for the normal development of the human immune system, thereby disrupting immune regulation.

The Hygiene Hypothesis: Sterile Environment, Excessive Immunity (Generated by Artificial Intelligence)

Historical Development and Key Observations

The foundation of the hygiene hypothesis lies in epidemiological observations that have examined various environmental and demographic factors influencing the prevalence of allergic and autoimmune diseases.

Family Size and Birth Order

Strachan’s original study showed that children growing up in larger families were less likely to develop hay fever than those with fewer siblings. This observation has been confirmed in numerous subsequent studies, leading to the hypothesis that older siblings may protect younger ones by exposing them to more infections early in life. This protective effect has been reported to be more pronounced when older brothers are present.

Childcare Centers (Daycares)

Attending daycare in early life increases children’s contact with other children and thus their exposure to infections. Some studies have shown that children who attend daycare during their first year have a reduced risk of asthma, atopy, and hay fever later in life. However, findings on this topic are inconsistent; some research reports no protective effect of daycare attendance and even suggests it may increase hay fever symptoms in certain cases.

Rural and Farm Life

Epidemiological data consistently show lower rates of allergic diseases and asthma among children raised in rural areas, particularly on farms. This protective effect is thought to result from intense exposure to microorganisms, especially endotoxins—components of the cell walls of gram-negative bacteria—through contact with farm animals, barn environments, and consumption of unpasteurized milk. It has also been reported that maternal exposure to a farm environment during pregnancy enhances the protective effect in the child.

Geographic Distribution and Migration Studies

The geographic distribution of allergic and autoimmune diseases provides another body of evidence supporting the hygiene hypothesis. The prevalence of these diseases shows a clear north-south gradient in North America and Europe, with higher rates in more industrialized northern countries. Similarly, lower prevalence rates in Eastern Europe compared to Western Europe have increased alongside rising living standards. Migration studies reinforce this observation: children from countries with low rates of allergic and autoimmune diseases who migrate to countries with high rates develop risk levels comparable to those of the host population.

Theoretical Approaches and Immunological Mechanisms

The biological mechanisms underlying the hygiene hypothesis have evolved alongside advances in our understanding of immune system regulation.

T-Helper Cell (Th1/Th2) Balance

The initial immunological explanation of the hypothesis was based on the balance between two subsets of T-helper (CD4+) cells: Th1 and Th2. According to this model, at birth and during the first months of life, the immune response is Th2-skewed. Bacterial and viral infections encountered in early childhood stimulate Th1 cells. Cytokines such as interferon-gamma (IFN-γ) secreted by Th1 cells suppress the activity of Th2 cells, which are responsible for allergic reactions. Inadequate microbial exposure in hygienic environments weakens this Th1 stimulation, leading to a persistent Th2 bias and increased susceptibility to allergy. However, this model fails to explain the rising incidence of autoimmune diseases, which are also mediated by Th1 responses.

Regulatory T Cells (Treg) and the “Old Friends” Hypothesis

More recent approaches center on regulatory T cells (Treg), which play a crucial role in immune regulation. This view holds that modern lifestyles have reduced exposure not only to common childhood infections but also to specific microorganisms that have co-evolved with humans throughout evolutionary history and are recognized by the immune system as “harmless”—termed “Old Friends.” These include saprophytic mycobacteria found in soil and untreated water, helminths (parasitic worms), and lactic acid bacteria such as Lactobacillus and Bifidobacterium that are part of the gut microbiota. These “Old Friends” directly stimulate the immune system to promote the development of Treg cells, which suppress both excessive Th1 (autoimmunity) and excessive Th2 (allergy) responses. These cells maintain immune balance by secreting regulatory cytokines such as interleukin-10 (IL-10) and transforming growth factor-beta (TGF-β).

Microbiota and Innate Immunity

The microbiota, a community of hundreds of different microbial species living in the human gut, plays a fundamental role in the development and education of the immune system. Modern dietary habits, antibiotic use, and cesarean delivery are thought to alter the diversity and composition of the gut microbiota. Studies have shown that the gut flora of allergic children differs from that of non-allergic children; for example, they have lower levels of bacteria such as Lactobacillus and Bifidobacterium and higher levels of bacteria such as Clostridium and Staphylococcus aureus. These microorganisms are recognized by innate immune receptors such as Toll-like receptors (TLRs), and this interaction is a critical step in activating regulatory immune pathways.

Criticism of the Hypothesis and Alternative Views

Although widely accepted, the hygiene hypothesis faces some contradictory findings and criticisms:

• Timing Issue: The decline in major infectious diseases such as typhoid and tuberculosis began well before the sharp rise in allergic diseases in the 1960s and 1970s. This makes it difficult to establish a direct temporal link between the reduction of these specific infections and the allergy epidemic.

• Role of Infections: Some studies have shown that certain infections may increase rather than decrease allergy risk. For example, respiratory syncytial virus (RSV) infection in infancy is associated with an increased risk of wheezing and asthma later in life. This indicates that not all infections are protective and that the effect depends on the type of infection, its timing, and host factors.

• The Concept of “Over-Cleanliness”: Although the popular interpretation of the hypothesis often links it to “over-cleanliness,” research shows that modern homes are not microbiologically sterile. Microbes continuously enter homes through food, pets, and people, and routine cleaning practices often fail to eliminate microbial exposure entirely. Therefore, the problem is not general hygiene standards but rather reduced exposure to specific “Old Friend” microorganisms that train the immune system.

Due to these criticisms, it has been proposed that the hypothesis be renamed “Microbial Deprivation Hypothesis” or “Old Friends Hypothesis.”

Applications and Therapeutic Approaches

The hygiene hypothesis has inspired new approaches to the prevention and treatment of allergic and autoimmune diseases.

Probiotics

Studies suggest that probiotics containing beneficial bacteria such as Lactobacillus and Bifidobacterium, particularly when used during pregnancy and infancy, may reduce the risk of atopic eczema. However, findings in this area are inconsistent, and further research is needed.

Helminth Therapy

Clinical trials have shown symptom improvement in inflammatory bowel diseases such as Crohn’s disease when controlled doses of pig whipworm (Trichuris suis) eggs are administered to patients.

Microbial Extracts and Vaccines

Research is underway to investigate whether vaccines or bacterial lysates derived from harmless mycobacterial species can modulate the immune system and reduce the severity of allergic diseases.

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