Is there a dirty side to hygiene?
Western populations today appear to be having the highest incidences of allergies ever. The ‘Hygiene Hypothesis’ has skirted around emerging findings regarding modern-day immune systems. But, is it real? Or are there other factors that are causing these inconvenient problems?
There are more cells of your being that are micro-organisms, than there are cells that are you. Over 1,000 different species of bacteria live on our skin. 500 to 1,000 microbes inhabit our gut, and as many as 2,000 could be in the air we breathe at any given time. Many of these are essential to our survival. Living in a commensal relationship, we all survive as unique species that rely on each other for various products or protection. For example, the magnitude of microbes that live on our skin make it almost impossible for an intruder to find enough space to establish themselves. But, there are many microbes that cause disease.
The immune system 101:
Our immune system can be split into two parts. Most aspects of the immune system work with some sort of receptor-recognition system. That is, the cell or protein detects something as foreign, and subsequently acts upon it, in whatever capacity it is supposed to. The first arm of the immune system to respond is known as the innate immune system. These cells and proteins have a broad range of receptors that detect dangerous or invasive things – they are non-specific. However, they do respond very quickly. The second arm, which normally takes a couple of days to hit the site of infection, is the acquired immune system. This aspect has incredible specificity to a particular microbe. The acquired immune system also has the capacity to generate ‘memory cells’. So, for some illnesses, if the infection recurs, the body mounts a quicker and more specific immune response.
When the immune system stuffs up, we can get allergies.
There is a unique aspect of the acquired immune system specific for parasitic infections. Antibodies, uniquely shaped proteins, are produced by the acquired immune system. Specifically the class called immunoglobulin E (IgE) for a parasitic infection. These antibodies attach to the parasite, and amongst other things trigger the release of proteins from other cells, and tag the parasite for destruction. Allergies occur when the body recognises innocuous molecules from the outside world. IgE binds much more tightly to particular immune cells than any other class of immunoglobulin. When an allergen first enters the body, the immune system prepares for another offense by binding IgE to immune cells. Upon a subsequent encounter with the allergen, the patrolling cells bound with IgE release histamine and other inflammatory molecules within seconds or minutes. These responses are obviously vital if a person is infected by a dangerous or lethal parasite. However, the effects of pesky sniffling and itchy eyes, to potential anaphylaxis are quite inconvenient, to say the least.
*Drum roll* The Hygiene Hypothesis:
Ok, so now we understand how allergies work. But, why are papers reporting increasingly higher numbers of allergies in the developing world. The ‘Hygiene Hypothesis’ was first proposed in 1989 by epidemiologist David Strachan. The theory suggests that a more modern lifestyle is more hygienic, and so children are less exposed to infectious agents, and thus their immune systems aren’t ‘trained’ and later backfire with allergic responses to unharmful molecules. However, hygienic living isn’t a new thing. But, we’re seeing rises in allergies now. The UK has quadrupled in consultations for asthma in the last 20 years, and the US has seen an increase in food allergies by 50% in the last 12 years. Furthermore, rapidly developing countries like China are seeing rising rates of allergies such as asthma.
So, what on Earth is going on?
Various other theories have been proposed following Strachan’s time. The ‘Counter-regulation’ hypothesis takes into account regulatory cells of the immune system. The theory suggests that in cleaner regions of the world, these regulatory cells aren’t trained properly, and have downstream effects on immune responses. What has been proven, is that younger children tend to get fewer allergies than their older siblings, and that children that grow up in big families or on farms also tend to develop fewer allergies. So, a key factor in growing a healthy immune system is spending time outdoors and with other children. Exposure to lots of different things to the immune system allows it to learn what it should and shouldn’t freak out about.
Scientists are now theorising about the perfect ‘window’ of opportunity to expose young ones to common allergens, such as peanuts. What has come out of most studies, is that babies that are born via Caesarian section, live in incredibly clean houses with little time spent outdoors or with other children, and who aren’t breastfed are most likely to struggle with allergies later on in life.
Keep on the lookout for new findings in this space!