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Researchers at the Universities of Zurich (UZH) and Lausanne (UNIL) have now scientifically proven in a study what many people have been suspecting for a long time: We are not doing our children any favors by shielding them from everything, especially anything that might make them ill. The human immune system forms by “training” during childhood.

The so-called hygiene hypothesis provides a widely regarded perspective. According to this hypothesis, improved hygiene, changes in agriculture and urbanization are to blame for the fact that our immune system comes into contact with certain microbes less often or later in life than before, making us more susceptible to diseases. This would increase chronic inflammatory diseases, allergies, and mental disorders such as depression.

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But not only too much hygiene, but also traumas experienced in childhood and allergies could trigger chronic inflammatory diseases and psychological impairments in adulthood. The Swiss scientists were able to prove these assumptions by identifying the early immune system programming of five groups of people.

One in five people has a very resistant immune system

The researchers based their study on the hygiene hypothesis and analyzed the epidemiological data of almost 5,000 people born in the mid-20th century. They concentrated on the coincidence of allergies, viral and bacterial diseases, and psychosocial stress in childhood. The scientists identified five different groups that they characterized by biomarkers (white blood cell counts, inflammatory markers). In a further step, they associated them with chronic inflammatory diseases and psychological disorders during adulthood.

The results showed that 60 percent of the individuals examined had an ordinary, “neutral” immune system and that their disease burden in childhood was comparatively low. 20 percent of the cohort exhibited a particularly resistant, “resilient” immune system. “Even symptoms of common childhood diseases like measles, mumps or rubella, which were not preventable in the mid-20th century, appeared far less frequently in this group than in the “neutral” group,” the researchers write.

On the other hand, there were also three smaller groups: 7% of the individuals, the “atopic” group, showed multiple allergic diseases. The “mixed” group (around 9%) had single allergic disorders. They suffered, for example, from drug allergies, bacterial and rash-inducing childhood diseases like scarlet fever, pertussis or rubella. The smallest of the five groups (around 5%) consisted of people who were traumatized in childhood. These people were more susceptible to allergic disease but were relatively resistant to typical viral childhood viral diseases.

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Hygiene hypothesis confirmed

An interesting and important point of the study is that the neutral and resilient groups are larger among people with earlier birth years than they were among individuals with later birth years. In contrast, the “atopic” group, i.e. those people who are more likely to show hypersensitivity reactions, has increased among younger cohorts. “Our study thus corroborates the hygiene hypothesis,” lead author Vladeta Ajdacic-Gross from the University of Zurich says, “but at the same time goes beyond it.”

Differences between the groups would manifest themselves also in later health. People of the “resilient” group were not only better protected against chronic inflammatory diseases in adulthood, but also against mental disorders. Members of the “atopic” and “mixed” group had an increased risk of disease as adults, both physically and mentally. In the “traumatized” group, people were more susceptible to mental illnesses in adulthood, and the women also had a higher risk of chronic inflammatory diseases.

“The findings of the study indicate that the human immune system acts like a switchboard between somatic and psychic processes,” Ajdacic-Gross explains. “They help us understand why many people who do not have a history of psychosocial trauma get afflicted by mental disorders and, conversely, why traumatized people show a predisposition to chronic inflammatory diseases.”

The results of the study “A step beyond the hygiene hypothesis – immune-mediated classes determinded in a population-based study” were published in the online journal BMC Medicine.