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Your Immune System is Not a Muscle

https://rachel.fast.ai/posts/2024-08-13-crowds-vs-friends/>

«Our immune systems are amazing, and amazingly complex. Certain cells, called memory B cells and memory T cells, are able to “remember” invaders that they have seen before.

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Is this an example of infection making you stronger? It depends. You can only catch the measles virus once, because you will form memory cells for it. But measles also destroys your pre-existing memory cells, meaning that you can now re-catch a bunch of other illnesses that you had already built immunity to.

[…] humans co-evolved with parasites and commensal bacteria, going back to when people lived in small hunter-gatherer tribes. For over 90% of human history, we lived as hunter-gatherers, and were exposed to very different types of microbes than we are now in crowded cities or poorly ventilated office buildings. Researchers have pointed out that the moniker Hygiene Hypothesis is misleading, and have proposed a more accurate alternative. The “Old Friends” mechanism describes which microbes we co-evolved with for 300,000 years.

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Surprisingly, there are a number of studies showing that infections with helminths (parasitic worms) can be beneficial in treating allergies and autoimmune disease, including for multiple sclerosis. A proposed mechanism is that parasites provide a low-level of background activation for the immune system, which prevents excess activation. This in term can prevent inflammatory and autoimmune disorders. However, helminth infections can also be harmful and are not something you should try at home! Researchers are developing therapies based on proteins derived from helminths.

Our “old friends” are organisms we co-evolved with for > 50,000 years. By evolving together, our bodies learned to take advantage of these organisms. Old friends can be contrasted with “crowd infections” which only developed much more recently, since 10,000 BCE, after people began to live in more densely populated townships.

Our immune systems did not evolve for this!

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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.

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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.»