The old friends hypothesis

Summary

This narrative review by Graham Rook, a leading proponent of the old friends hypothesis, argues that mammals have co-evolved with a suite of microorganisms — including environmental bacteria, helminths and certain commensals — that are essential for appropriate immune calibration. Rook's hypothesis, published in Frontiers in Allergy, posits that modern reductions in exposure to these organisms, driven by urbanisation, sanitation, dietary change and loss of contact with biodiverse environments, underlie the epidemic rise in allergic, autoimmune and inflammatory conditions. The paper likely synthesises immunological and epidemiological evidence in support of this framework and discusses implications for prevention and therapy.

UK applicability

The old friends hypothesis has direct relevance to UK public health policy, given rising rates of allergy, asthma and autoimmune disease in the UK. It also has implications for land use and farming systems research, as biodiversity-rich agricultural and natural environments may be important sources of beneficial microbial exposure for rural and peri-urban populations.

Key measures

Immune regulatory markers; prevalence of allergic and inflammatory conditions; microbiome diversity indices; epidemiological trends in non-communicable inflammatory disease

Outcomes reported

The paper examines how reduced exposure to co-evolved microorganisms ('old friends') — including helminths, environmental bacteria and commensal organisms — impairs immune regulation and increases susceptibility to inflammatory and allergic conditions. It likely reports on epidemiological and mechanistic evidence linking biodiversity loss, lifestyle changes and rising rates of immune-mediated disease.

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