Unraveling the Myth: Does a “Squeaky Clean” Environment Weaken Our Immune System?

Introduction

Since time-immemorial, the human immune system has been co-evolving alongside a diverse array of microorganisms that are omnipresent in our ancestral environment. Such microbes are tolerated by the immune system and become crucial for training regulatory T cells (T_reg), which prevents the immune system from overreacting to harmless antigens. 

Nevertheless, there has been a surge in allergies especially among children in developed nations over the past few decades. Coincidentally, civilisation has been escalating in parallel. This raises suspicion, has our temporal pursuit for a germ-free household gone too far?

The “Old Friends” Theory

That’s what the “old friends” theory is about. First coined by Graham Rook, he realised that it’s the early and regular exposure to harmless microorganisms - “old friends” - which go on to drive our immune development, training the immune system to react appropriately in the face of threats as a consequence. From there, this turned into a full-fledged postulation that a broad failure of immunoregulation looming nowadays actually has a causative factor in an inadequate exposure to such microbes.

On account of socioeconomic development, most contemporary urban dwellers are no longer living in run-down hovels. Associated with improved sanitation, cholera and typhoid - once major causes of mortality in Europe - has dwindled drastically by the late 19th century. On the other hand, occurrences of allergic rhinitis have just risen to an unprecedentedly high level. People 100 years ago are generally nowhere near as attuned to dust, pollen and fur as the population today. How could this be?

The “Hygiene Hypothesis”

A seemingly correlation between hygiene enhancement and allergy increment - be it a coincidence by mere chance or by obscurities lying within - led Strachan into scrutiny. This gives birth to his “hygiene hypothesis”. At first, he proposed an inverse relationship between family size and the prevalence of atopic diseases. Nonetheless, this was debunked after extensive research as it becomes clear that atopic diseases are plummeting on the ground of hygienic measures alongside other confounders in lieu of a literal diminish in family size.

In the meanwhile, the catchy term of “hygiene hypothesis” almost immediately became popular among the public. The media and scientific community loved this notion, and it was soon extrapolated to include other modern changes in personal hygiene. Considering the theoretical interpretation that holds validity, things might seem fine at first glance.

Nevertheless, such a phrase of “hygiene hypothesis” turns out to be a dangerous misnomer, as the word “hygiene” often erroneously prejudges whatever the confounding factors are. People started to show skepticism towards sanitation. Yet, simply ditching hand sanitisers and casting soaps away will just save nothing but make matters worse by possibly bringing back typhoid. In reality, what is happening in our temporal world of rising allergies is elicited by multifaceted factors which can never be pinpointed into just one root per se.

The big picture: Why are allergies frequent among children these days?

Let’s look at the changes within the near 100 years.

Factor	Mechanism
Climate Change	Climate change elicits longer and more intense pollen seasons, which exacerbates seasonal pollen allergies.
Pollution	Air pollution can aggravate allergy symptoms.
Indoor Lifestyle	People nowadays are more exposed to dust mites, pet dander and mold spores, which are major causes of allergic rhinitis. In comparison to homes in previous centuries, modern homes are often sealed tighter, trapping these allergens inside.
Urbanisation	Children are exposed to fewer germs in early life. This lack of exposure prevents the immune system from training itself properly, rendering it to overreact to innocuous substances like dust or pollen.
Dietetic Change	This alters gut microbiomes, which play a role in regulating allergic inflammation as well.

This table aims to deliver a candid overview for the public to acknowledge that our issue on allergies is caused by a myriad of different factors affecting one another. After all, microbial exposure remains one of the nexus between exogenous factors and our body’s immune responses. For a critical discussion, we will just focus on altered microbial exposure onwards.

Comparison of High and Low Exposure Environments

Admittedly, the exposure of city kids nowadays to the microbial world is far more circumscribed than it once was. 

Feature	High Exposure Environments (e.g. farms)	Low Exposure Environments (e.g. modern household)
Microbial Diversity	High	Low
Sources Mediating Exposure	Livestock, raw milk, soil, pets, siblings	Sanitisers, antibiotics, detergents, closed air-conditioned systems
Immune System Impact	Develops tolerance	Hypersensitivity

Immunological Mechanisms

The expression of allergic phenotype is shaped by two different factors: a genetic predisposition and environmental interactions. Yet, the recent rise in allergies has occurred within too short a time frame to be explained by a genetic shift in the population. Hence, exogenous influences are what researchers look into. In the context of a diminishing microbial exposure, three immunological mechanisms have been proposed to account for its influences on allergy.

Prior to diving into them, let’s take a look at the key players: Th1, Th2 and regulatory T cells (T_reg). In simple terms, allergies are driven by Th2 cells, which react strongly to harmless substances like pollen or dust mites. Microbes and their products (via Toll‑like receptors on innate immune cells) usually promote Th1 responses, which counterbalance Th2. Regulatory T cells (T_reg) act as brakes, broadly damping excessive immune responses, including both Th1 and Th2. So, here are the mechanisms:

1. Missing immune deviation: At birth, the immune system is naturally biased toward Th2. With repeated microbial stimulation, it normally deviates toward more Th1‑type responses as children grow. Well, this happens because microbes trigger dendritic cells and NK cells to release cytokines such as IL‑12 and interferons, which push T cells toward a Th1 profile and simultaneously oppose Th2 development. In the event of a reduced microbial exposure in early life, the lack of such a shift from Th2 dominance to Th1 dominance keeps the immune system skewed toward allergies.

2. Reduced immune suppression:

3. Failed tolerogenic environment: Infections can boost regulatory T cells (T_reg) cells and suppressive cytokines like IL‑10 and TGF‑β, globally calming immune responses. Insufficient exposure to microbes prevents the immune system from establishing a “tolerogenic” environment, causing it to overreact to harmless antigens.

4. Reduced regulatory T cells (T_reg) Generation: Microbial metabolites (e.g., short-chain fatty acids from commensal bacteria) normally drive the differentiation of regulatory T cells (T_reg). Less exposure means fewer regulatory T cells (T_reg) are generated.

5. Chronic Inflammation: Without adequate T_reg-mediated suppression, the body experiences raised background levels of inflammation (e.g. high C-reactive protein), which is associated with autoimmune disorders, asthma and inflammatory bowel disease.

3. The biodiversity theory of allergy: Environmental microbiota - from soil, plants, animals and such - is pivotal to enrich human microbiome, thereby driving immune maturation and tolerance by maintaining regulatory T cells (T_reg). This process tempers allergic sensitisation. Therefore, it is inferred that a lack of exposure to natural microorganisms can hamper such mechanisms, putting children at stake of immune aberrations favouring excessive inflammatory responses.

There is mounting research done on the first two mechanisms in the early 21st century, but such studies eventually reach a plateau when gaps in our understanding towards Th1, Th2 and regulatory T cells (T_reg) fail to be redressed. Soon, investigations with regard to missing immune deviation fell out of favour, whereas latest research has shed new light on the roles of regulatory T cells (T_reg). The latter mechanism involving human microbiota is a relatively fresh viewpoint that has garnered substantial attention nowadays.

The Gut-Lung Axis

Our diet plays a key role in shaping our intestinal microbiota. These are beneficial microbes that reside within our gut, helping us out in various aspects, immune system inclusive. When our diet changes, this modulates our gut microbiota, which in turn, may have a long-reaching effect by posing an impact on respiratory disease manifestations. That’s what is discussed as per the gut-lung axis.

To illustrate, short-chain fatty acids (SCFAs) produced by bacterial fermentation on dietary fibres within the gut are crucial immune modulators that reduce inflammation. They may translocate across the intestinal barrier to reach the systemic circulation via mesenteric lymphatic system, eliciting an influence on lung immune response as a consequence. In lungs, it is shown that SFCAs act as signalling molecules on resident antigen-presenting cells (APCs) to attenuate inflammatory and allergic responses. On the underpinnings of such understanding, it is thereby inferred that a low-fiber diet can indirectly increase the susceptibility of an individual to respiratory allergic sensitisation.

Practical Tips for a Healthy Balance

1. Encourage outdoor play and interaction with nature: Like it or lump it, epidemiological studies comparing rural with urban children consistently consolidate the hypothesis that exposures to natural environments are correlated to a reduction in allergies. Delving deeper, such a phenomenon might be owing to sufficient immune training, less pollution, greater psychological wellbeing, or anything in between. Theories aside, just immerse yourself in the lush greenery, and you will feel much better!

2. Prioritise a fibre-rich diet: Wholegrain breads - which are firm and nutty - may fall short in terms of being savoury, but they are exactly what most of us need! Fuel up yourself with a diet rich in dietary fibres (e.g. legumes and wholegrains), and you will likely be conferred with a healthy gut microbiota as well as an upboost in immune health.

Challenges and Limitations

1. Knowledge gap in immunology: For decades, scientists have been postulating a few molecular mechanisms implying the effects of reduced early microbial exposure to the immune system. Nonetheless, our current gap in knowledge with pertinence to Th1, Th2 and regulatory T cells (T_reg)  poses skepticism on the cogency of such theoretical interpretations.

2. Lack of quantifiable measurements: Epidemiology aside, there is a lack of quantitative scientific data providing empirical validation. It remains unclear to what extent might modern cleaning, hygiene and dietetic alterations influence the reduction in “old friends” exposure, and to what extent does circumscribing such an exposure elicits a rise in allergies.

Conclusion

While the quest for a germ-free environment was born out of a necessary battle against contagious diseases, the extrapolated “hygiene hypothesis” has somehow suggested that our modern, sanitised lifestyle comes with an immunological cost. In the meanwhile, pediatric allergies demonstrate an upsurge these days, prompting the public to speculate a correlation between the two. Nevertheless, simply blaming the issue on hygiene per se is a sweeping generalisation that is foolhardy, as the rising prevalence of allergies among children today is not the result of hygiene alone, but a sophisticated interplay of climate change, urbanisation and dietary shifts.

On the basis of current evidence, what we know has been justified is that a dwindling exposure to the natural microbial world does pose implications on our immune system, rendering this to be a significant intermediary connecting a host of exogenous factors to rising allergies. As such, it will be sagacious for us to support our immune health by regularly interacting with nature and maintaining a fibre-rich diet whilst awaiting the scientific community to close the gap within theoretical interpretations and empirical validations pertaining to this context.

Citation

1. ‌Talha, M., Rehman, U.U., Fatima, M., Gill, O.A. and Saifullah, M. (2025). The biodiversity theory of allergy: childhood exposure to natural environments and allergy risk. Annals of Medicine & Surgery, [online] 88(1), pp.984–986. doi:https://doi.org/10.1097/ms9.0000000000004328.

2. ‌Enaud, R., Prevel, R., Ciarlo, E., Beaufils, F., Wieërs, G., Guery, B. and Delhaes, L. (2020). The Gut-Lung Axis in Health and Respiratory Diseases: A Place for Inter-Organ and Inter-Kingdom Crosstalks. Frontiers in Cellular and Infection Microbiology, [online] 10. doi:https://doi.org/10.3389/fcimb.2020.00009.

3. Bloomfield, S.F., Stanwell-Smith, R., Crevel, R.W.R. and Pickup, J. (2006). Too clean, or not too clean: the Hygiene Hypothesis and home hygiene. Clinical Experimental Allergy, [online] 36(4), pp.402–425. doi:https://doi.org/10.1111/j.1365-2222.2006.02463.x.

4. Romagnani, S. (2004). Immunologic influences on allergy and the TH1/TH2 balance. Journal of Allergy and Clinical Immunology, 113(3), pp.395–400. doi:https://doi.org/10.1016/j.jaci.2003.11.025.

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Aritcle written by Low Yi Xian

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