The Great Debates: Parasites and Immune Function

In industrialized societies, the rise of autoimmune diseases, allergies, and chronic inflammatory conditions has prompted scientists to reevaluate the role of environmental exposures in immune system development. One particularly compelling area of study is the impact of parasite exposure on the human immune system. Contrary to modern assumptions that parasites are purely harmful, emerging research suggests that certain levels of parasitic exposure during early life play a crucial role in calibrating immune responses, ultimately protecting against chronic diseases. This is becoming fairly similar to the attitudes about bacterial infections in the 1950's. At that time bacteria were thought to be only pathogenic, and needed to be eradicated. There are many examples of this happening particularly in the alternative medicine field. Candida albicans is a great example of how a particular organism can become vilified by the mainstream influencers and create an almost hysteria about irradiation from the body. However, this particular organism is part of the human gastrointestinal flora and is needed in regulated amounts in the g.i. tract. It cannot and should not be completely eradicated from the body. These organisms have been on the planet longer than our species, and because of that fact it must be acknowledged that we have evolved along side these organisms. Just as the human gastrointestinal tract evolved along side the bacteria that now make up the symbiotic relationship between human and pathogen, parasites too have evolved with us to form useful partnerships that now are an intricate part of the immune systems balance. The lifestyle of the Tsimané people of the Bolivian Amazon offers powerful insight into this relationship.

The “hygiene hypothesis” was first proposed in the late 20th century to explain the increasing prevalence of allergic and autoimmune conditions in industrialized nations. It posits that reduced exposure to infectious agents—especially in early childhood—leads to an under-developed immune system that may overreact to benign substances, resulting in conditions like asthma, eczema, Crohn’s disease, and multiple sclerosis. This concept has evolved into the “old friends hypothesis,” which emphasizes the evolutionary importance of exposure to certain microbes and parasites (such as helminths) that coexisted with humans for millennia. These organisms helped regulate our immune systems, keeping inflammatory responses in check and supporting the development of tolerance to harmless antigens.

The human immune system comprises two major branches: the innate immune system, which provides general defense against pathogens, and the adaptive immune system, which targets specific threats. Chronic exposure to parasites—particularly helminths—appears to fine-tune both systems in several ways:

1. Regulatory T Cell Development: Parasites stimulate the production of regulatory T cells (Tregs), which suppress overactive immune responses and reduce the risk of autoimmune disease.

2. Th2 Dominance: Helminths often shift the immune system toward a Th2 (anti-inflammatory) profile, counterbalancing the pro-inflammatory Th1 and Th17 responses implicated in autoimmune and inflammatory diseases.

3. Immune Tolerance: Exposure to a diverse array of antigens teaches the immune system to tolerate non-harmful substances, reducing allergic reactions.

To find examples of this in modern life, one must look in the undeveloped regions of the world. One case study example is a tribe in the Amazonian jungles of Bolivia called the Tsimané. This is a tribe of indigenous forager-horticulturalists that lacks almost any exposure to modern developed life. Their lifestyle includes high exposure to a wide range of microbes and parasites, largely due to their subsistence diet, lack of modern sanitation, and frequent contact with the natural environment. Paradoxically, despite carrying high loads of parasitic infections, they experience very low levels of chronic inflammatory diseases common in Western populations.

A landmark 2017 study published in The Lancet found that the Tsimané had the lowest levels of coronary artery calcification ever recorded, despite poor access to modern healthcare. In addition:

• Parasitic Infections Are Widespread: Over 70% of Tsimané individuals test positive for at least one intestinal helminth.

• Reduced Chronic Inflammation: They exhibit chronically elevated levels of certain immune markers, such as eosinophils, but these are not associated with harmful inflammation or tissue damage. Instead, they may reflect a balanced, active immune system.

• Lower Incidence of Autoimmune Conditions: There are virtually no reported cases of autoimmune diseases, such as type 1 diabetes or multiple sclerosis, within the Tsimané population.

• Immune System Composition: Their immune systems show a bias toward regulatory and anti-inflammatory responses, likely influenced by persistent low-level parasitic exposure.

This research has many implications for modern health, especially in the holistic health and wellness landscape. The Tsimané experience suggests that immune system development is optimized in environments rich in microbial and parasitic exposures—conditions vastly different from sterile urban settings. While the goal is not to reintroduce pathogenic infections, these findings do encourage a rethinking of hyper-hygienic practices and excessive antibiotic use.

There are also potential therapeutic strategies inspired by this research. Things like probiotic and microbial exposure therapies, controlled helminthic therapy trials for autoimmune diseases such as ulcerative colitis and multiple sclerosis, and even rewilding environments, especially during early childhood, to increase safe microbial exposure and full immune system development.

Our immune systems did not evolve in a vacuum—it developed in constant conversation with a rich array of microorganisms, including parasites. We have seen this through our understanding of our own microbiome and the microbiota that it is composed of. The evidence from the Tsimané people underscores the importance of this evolutionary relationship. Because of the side by side evolution with these organisms and the human body they should not be considered part of the "modern illness paradigm", like many of the other chronic disease contributors. Controlled exposure to parasites and other microbes may actually be a key to preventing the chronic inflammatory diseases that plague modern societies, not the cause of them. As we design future health strategies, incorporating this ancestral perspective on immunity could lead to a more balanced and resilient immune system for all.

References

Kaplan, H. et al. (2017). Coronary atherosclerosis in indigenous South American Tsimané: a cross-sectional cohort study. The Lancet, 389(10080), 1730–1739.

Gurven, M. et al. (2009). Inflammation and infection in a population with minimal cardiovascular disease. Annals of Human Biology, 36(3), 229–242.

Rook, G. A. W. (2010). 99th Dahlem conference on infection, inflammation and chronic inflammatory disorders: Darwinian medicine and the ‘hygiene’ or ‘old friends’ hypothesis. Clinical and Experimental Immunology, 160(1), 70–79.