the “Old Friends” immunology hypothesis
How the “Old Friends” hypothesis is taking over from the hygiene hypothesis:
Homo Sapiens first evolved some 300,000 years ago, yet crowd infections are believed to have only developed in the last 12,000 years, a small blip in human history. Humans living in dense cities is a relatively recent development. An even more recent development is that of sealed indoor spaces and frequent international air travel. Many crowd infections, such as measles, mumps, chickenpox, colds, and flu, are airborne, spreading when humans talk and breathe in close contact, with poor ventilation. These infections could not widely spread until the last few hundred years of human history. When I began studying immunology, something that surprised me is how much of the immune system is focused on fighting parasites. There is an entire branch, including several cell types, devoted to this. It seems like such a mismatch to the modern, industrialized world. “Can I have a few more immune cell types focused on viruses or intracellular bacteria?” I thought, “in exchange for some of these parasite-focused cells that I’m not using?” Our “old friends” are quite different from the crowd infections that plague us now – it would be bizarre to assume that research based on one of these categories will apply to the other! Our “old friends”, parasitic worms and beneficial microbes, are associated with a reduced risk of allergies and autoimmune diseases. No such relationship exists for crowd diseases. In fact, the opposite is true. Crowd diseases contribute to allergies and autoimmune diseases. Comparing the immune system to a muscle that gets stronger with use is overly simplistic and, in many cases, inaccurate. There is huge variety in how various pathogens impact us. Being precise in considering different types of microbes and infections will allow us to better understand human health.
(tags: articles health medicine immunology old-friends hygiene-hypothesis allergies autoimmune disease parasites)