Why do we still have allergies if humans have lived around plants for a long time?
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I know an interesting answer to this! Male trees and other plants have been planted much more than female trees for the past 100+ years by humans. This is due to the fact that they do not fruit, and make less of a mess. What happens when you don’t plant any female trees? Pollen spreads everywhere without anywhere to go. That’s why we see trees covered in yellow pollen online. Nature didn’t intend for it to be this way. We just love to disrupt nature, and cause chaos. Some trees are both male and female, so they are kind of superior. Ultimately, we need to plant more female trees!
There are a lot of things that are still unknown about how immune systems work, but one thing we do know is that the things they attack are not “pre-programmed”. The body learns about them based on exposure and has to guess what it should attack and what it should not.
The problem is that we don’t live in the same way our ancestors did, so biological systems which worked well in the past cause errors today. By creating an environment with less risk of infection, we may have accidentally caused an increase in autoimmune disease. This is called the [hygiene hypothesis](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2841828/), and while it is backed up by correlative data (countries with less infectious disease experience more autoimmune disease and allergies), we don’t fully understand *why*.
One theory is that the immune system “expects” there to be a certain amount of pathogens in the environment. Since our environments are a lot cleaner than they used to be, the immune system gets antsy and starts firing away at anything unusual it spots.
Another possibility is that the immune system determines what’s “normal” based on what children are exposed to early in life. Since children are less likely to be outside than they used to be, the immune system doesn’t get the early exposure it needs and is more likely to freak out when it gets exposed to unfamiliar antigens.
There is also evidence that many common parasites, as well as some benign endosymbionts, actively produce hormones that suppress the immune system, and that we evolved to compensate for this by getting a more sensitive immune system. We’ve been in an arms race for such a long time that we have actually come to depend on parasites to keep our hyperactive immune system in check, and now that our environment has fewer parasites the immune system is more likely to go haywire.
This isn’t to say that hygiene is *bad*, mind you. The amount of suffering caused by pathogens in developing countries without modern medicine far outweighs the problems caused by autoimmune diseases in modern countries. But it does suggest that there are consequences to our overall improved standard of living which need to be addressed.
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