The modern world is constantly trying to eradicate bacteria and other pathogens and to get rid of dirt. Parents often think that visiting the local playground increases children's risk of contracting diseases and that staying at home without exposure to dirt and other kids is the best solution for keeping kids healthy. Before turning on the A/C at home and locking the door please read the following article in which researcher Rotem Sisso, gives a thorough explanation about the hygiene hypothesis and the relation between exposure to bacteria and microorganisms and the strength of our immune system.
The hygiene hypothesis

Our immune system operates as a barrier protecting us against harmful substances and microorganisms such as bacteria, viruses and other allergens. During childhood, our immune system matures and learns how to distinguish between harmful and non-harmful elements. In 1989, Dr. David Strachan was the first to suggest the hygiene hypothesis. According to this theory, a lack of early exposure to different microorganisms can compromise the healthy maturation of the immune system. Therefore, growing up in an environment too sterile, without exposure to diseases, does not allow the immune system to adapt and mature properly. As a result, the immune system becomes overly sensitized and later in life recognizes normal elements as harmful. This leads to the development of allergies and other childhood diseases that are caused by abnormal activation of the immune system, such as asthma.

Microbial diversity

Despite the fact that the hygiene hypothesis is supported by epidemiologic studies, advances in research such as genomic sequencing and bioinformatics lead to the understanding that early exposure to infections is important but that it is not the only factor causing the development of diseases such as asthma. These studies showed that there is great importance in having a diverse commensal microbiome (for our previous article regarding microbiome- click here). 

Evidence from many studies have demonstrated that different exposures, in addition to infections, can change our microbiome and contribute to the development of asthma and allergy. These exposures include the type of birth (vaginal versus caesarian), breastfeeding, exposure to pets, attending daycare and antibiotic treatments during early childhood.

The country mouse and the city mouse

The “farm effect” is one of the most convincing observations arising from microbiome studies in the context of asthma and is a key phenomenon that kept that hygiene hypothesis relevant. Across the world, observational studies have shown that people that grew up in farms have lower rates of asthma and other allergic diseases. In a study published in 2019 in the leading journal, Nature, researchers compared the bacterial population in dust taken from farm-houses, houses in villages without agricultural activity and houses located in the suburbs. The dust microbiome in both types of village houses was similar and babies who grew up in such houses had lower rates of asthma compared to babies who grew up in the suburbs.

How much should we expose the children to allergens?

The hygiene hypothesis offers an interesting perspective on the relation between over-cleanliness, reduced exposure to microorganisms and the development of childhood diseases, specifically allergies and asthma. Therefore we ask ourselves what is the right amount of exposure and which exposure might risk children?

Like everything in life balance and moderation is key. By adopting a balanced approach to this issue you can help your children build up a healthy immune system that can deal with various challenges. Encourage your child to play outside, with their siblings and other friends in playgrounds and field trips. Remember that pets, especially dogs and cats, further diversify our microbial environments. And most importantly, don’t be afraid to let your kids get a little bit dirty.

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