Feature: How Evolution Can Explain Allergies
By on September 16th, 2012

Every summer, when I return home for my vacations, I am hesitant about eating food at roadside stalls because, you know, who knows how unhygienic the food there is? In a role reversal that I still find amusingly ironic, my mother would accuse me of being wimpy and shove a plateful of food into my hands. Her logic—if we are over-protective of our immune systems, they will ‘forget’ how to respond when hit by a major infection. Blood them in battle, she said.

I’m still going to hold off from the delicious and teeming-with-microbes sugarcane juice on Indian roads, but as it turned out my mother was on the right track. A variant of her statement does apply in the case of allergies in what has been proposed as the ‘Hygiene Hypothesis’. Did you know that the incidence of autoimmune diseases (allergies being a prime example of these) is much higher in industrialized countries? Exposure to infectious agents in childhood primes your immune system for a more effective immune response as you grow up. Conversely, an extremely sanitized environment (often seen in industrialized countries) during childhood can make your immune system weak, unprepared to face infections and respond to harmless molecules that then become allergens.

Evolutionary Mismatch

How, and why does this happen? The answer lies in an ‘evolutionary mismatch’. Our bodies evolved in an environment which is very different from the one we live in now.

Let’s travel back in time for a little bit. In the first stage of human history, members of our species were hunter-gatherers. Our immune systems were constantly being exposed to a host of microbial organisms and worms. Around 12,000 years ago, we started settling down and took to agriculture. We continued being exposed to microbes, and in fact the sedentary lifestyle led us to being exposed to them for longer periods and increased human-human transmission. And then came the Industrial Revolution, bringing with it sanitation, vaccines and the beginning of the world as we know it. Many of the organisms that our ancestors encountered on a daily basis are now depleted from our present-day environment.

[Image Credit: ucla/ Nature Immunology]

 

‘The Old Friends’ Hypothesis’

We have grown up in the industrial age, but our immune system has evolved over centuries in the first and second stages of human history. Microbes and worms were so omnipresent that our immune systems learned to tolerate their presence in the body if they were harmless. Reacting to an infection is costly for the body, as we know from the all-pervading weakness we experience after a fever. A wiser route for the immune system was to just let the microbe exist, and simultaneously, the worms evolved to release certain molecules that would down-regulate certain components of the human inflammatory system. In the current environment, our bodies do not contain the micro-organisms that regulated our immune system. Our immune systems thus rise to inflammatory baits in a heartbeat.

It’s still a hypothesis, but there’s plenty of evidence that supports it. Guts of children with allergies have been found to have fewer numbers of a bacterial species called lactobacillus. Another study in Argentina showed that people with fewer worms called helminths have fewer incidences of multiple sclerosis (MS).

Our bodies are thus not adapted to the environments we live in, leading to this kind of a mismatch. In context of public health, it is not feasible to think of returning to the environments of our ancestors, nor is it feasible to think of infecting allergic patients with 50 hookworms that would downregulate the immune response. However, learning more about the symbiotic  mechanisms between our ‘old friends’ and our immune systems could help design more effective therapies towards autoimmune disorders.

Sources:

Microbes, immunoregulation and the gut

Old Friends Hypothesis

How Parasites can trick your immune system

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Author: Shweta Ramdas
Beginning life as a grad student studying human genetics.

Shweta Ramdas has written and can be contacted at shweta@techie-buzz.com.

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