A University of Nottingham study is holding freedom from louse infestation responsible for increased allergic reactions.
The finding means that the epidemic of allergic disorders in modern, urban people might be due to our having rid ourselves of lice and worms.
As per "hygiene hypothesis" humans' immune systems evolved to compensate for continual infections with parasitic gut worms, which secrete chemicals that reduce our immune responses.
People who are now worm-free have overreactive immune systems, which can lead to asthma and autoimmune disorders.
Yet mammals' immune systems have mainly been studied in clean, well-fed and parasite-free lab mice.
In the study conducted on wild wood mice, Janette Bradley and her colleagues have found that body louse reduced the readiness of the innate system to mount an immune response.
"Our understanding of mammalian immunology is largely based on rodents reared under highly unnatural pathogen- and stress-free conditions," said Bradley.
"Analysing immune responses in wild populations can give crucial insights into how the immune system functions in its natural context," she added.
During the study, the authors conducted post-mortem on the captured mice, assessing their weight, parasite load, and the responsiveness of their spleen cells to substances such as heat-killed listeria and bacteria, which bind receptors of the innate immune system and provoke a measurable reaction.
They found that those mice uninfected with the louse Polyplax serrata showed markedly increased responses to these triggers of innate immune responses, compared to highly-infected animals.
This suggests that the parasite is able to exert some kind of immunosuppressive effect, possibly directly by secreting some substance into the mice from its saliva, or indirectly by transmitting bacteria or other pathogens.
The authors speculate that this profound dampening of innate immune responsiveness supports the view that modern parasite-free human populations have a level of heightened immune responsiveness that would not have been typical during their recent evolutionary history,
"Much like laboratory mice, people in developed countries are currently exposed to a very different profile of infections to that encountered by their ancestors.
"It is possible that the immune dysfunctions we see today are the result of immune systems calibrated for a set of challenges completely different to those they now routinely face," she added.
The study adds to evidence supporting the so-called hygiene hypothesis, which holds that the rise in asthma and allergies can be linked to hyper-clean living.
The study appears in the BioMed Central journal BMC Biology.