Questions arise over why the prevalence of different diseases change. In the case of allergic diseases like asthma, we see the number of asthma affected children growing each decade. Given the impact of asthma on the person’s life, scientists want to know why more and more children are having to live with this disease. For years, the hygiene hypothesis has been thrown around. This is the idea that a lack of exposure to infections during early life increases allergic reactions including asthma. Basically, an excessively clean environment is suspected to influence the immune system towards an overreaction along allergic pathways.
Recent estimates put the rate of affected school aged children at 10%. Most first develop their symptoms at age 5 years or earlier. Increased mucous in the airway and spasms of the muscles around the airways lead to trouble breathing and the sound of wheezing. This can even be life threatening.
University of Alabama researchers used a mouse model to determine how mice responded to different levels of a bacterial chemical we know as lipopolysaccharide (LPS). They discovered that younger mice required a higher level of LPS exposure to prevent allergic reactions to house dust mites (a common cause of allergies). Without an adequate level of LPS exposure, say from bacteria, the young mice were more likely to develop mouse asthma. For adult mice, less LPS exposure was needed to prevent allergic tendencies.
How does this play into the hygiene hypothesis? Basically, this supports the idea that children who are exposed to less bacteria, i.e. less germs, end up having more allergic reactions. Rather than just settling for a correlation, the researchers determined a probable mechanism by which LPS could trigger such an allergic reaction. Naïve T cells, ones that had not yet encountered a foreign antigen, would turn into T helper type 2 cells more often when they did not encounter sufficient LPS. More LPS meant fewer T helper 2 cells and thus less allergic inflammation.
Now, we have more than just correlation studies like ones that indicated children on farms developed less asthma and children who washed dishes by hand developed less asthma. We have a probable mechanism. In both cases, those kids get exposed to more bacteria on average. It makes sense. Our immune system functions better when constant gentle stimulation from bacteria occur. Living in an overly sterile environment likely causes an imbalance and in this situation, more allergic diseases. To prepare for 2020 and Beyond, we need to strengthen and balance our immune systems with a little dirt.
Holly Bachus, Kamaljeet Kaur, Amber M. Papillion, Tatiana T. Marquez-Lago, Zhihong Yu, André Ballesteros-Tato, Sadis Matalon, Beatriz León. Impaired Tumor-Necrosis-Factor-α-driven Dendritic Cell Activation Limits Lipopolysaccharide-Induced Protection from Allergic Inflammation in Infants. Immunity, 2019; DOI: 10.1016/j.immuni.2018.11.012
Thanks to Science Daily
University of Alabama at Birmingham. “Mechanism for impaired allergic inflammation in infants may explain hygiene hypothesis: The hygiene hypothesis says increased sanitation in industrialized countries has led to increases in asthma.” ScienceDaily. ScienceDaily, 9 January 2019. <www.sciencedaily.com/releases/2019/01/190109110115.htm>.