Microbes & immunity – friend or foe?
A large part of what our immune system does is fight off germs to keep us well. But our understanding is actually much broader: Immunity ensures we respond appropriately when we encounter anything in the world around us — not only harmful germs. With the vast improvements in sanitation and healthcare, germs are no longer the enemy in our modern-day lifestyles. In fact, 99% of germs that surround us at any given moment (and we are surrounded everywhere all the time) do not cause disease. Rather, they are healthy ‘good’ bacteria — our microscopic health allies and immune system educators. Your immune system is all the time being calibrated by your body’s good bacteria to only respond to things that are actually threatening. Our relationship with ‘good’ microbes on us, in us and around means we are actually super ecosystems. But if that system breaks down it can lead to immune-mediated diseases such as allergy.
Time to abandon the hygiene hypothesis
Decreased exposure to infections in early life through improved ‘hygiene’ was originally thought to increase the risk of allergy. The idea that there might be a link between the rise in allergy and a decline in infection, known as the ‘hygiene hypothesis’, was proposed back in 1989 (4). Early childhood exposure to germs was thought to protect from allergy by encouraging proper immune development. But are allergies really the price we pay for freedom from the burden of killer infections that our grandparents feared yet are rare in our modern world?
True, proper development of our immune system is supported by interactions with germs but hygiene is a misnomer. The hygiene hypothesis has now been largely been overthrown, replaced by the ‘Old Friends’ Hypothesis (5). This suggests that rather than being too clean, the most important change in our environment that leaves us open to allergies the loss of contact with our ‘old friends’ – the many harmless microbes in us, on us and around us from birth.
Interacting with these ‘old friends’, are not the exposures to colds, flu or serious childhood diseases, but rather the 99% of harmless microbes in our environment. Our immune systems need to ‘see’ these good germs which in turn provide vital inputs that educate and shape our immunity from birth. This process serves to diversify our own flourishing microbial ecosystems which are in charge of training and shaping our immune systems, tinkering with how our immune system chooses to respond or ‘tolerate’ harmless things in our environment. Much of this happens in the microbes’ own backyard – our guts! In fact, these days, there is little doubt that your gut resident microbes, have a big say in how your whole immune system develops, including how whether you develop allergies or not (6).
So it follows that if your gut microbiota are in charge of training the immune system. When there are alterations in the composition of our gut microbiota, consequences like allergies might arise. But how? The gut microbiota is a changing ecosystem, containing trillions of bacteria, continuously shaped by many factors, such as dietary habits, seasonality, lifestyle, stress, antibiotics use, or diseases.