[munchbunny]

My oldest is allergic and spent plenty of time in the sand and dirt. They were exposed to nuts pretty regularly, and fed small amounts from basically as soon as they were taking solid foods.

The hygiene hypothesis is widely accepted, including by allergists, and there's definitely data supporting it, but we don't understand the mechanism, so it's hard to say that it's about any one specific thing vs. many contributing causes that correlate with hygiene and other aspects of the environment around the kid.

The advice about early exposure clearly works though, and there's data to support that early exposure even after confirming the allergy can increase the chances of outgrowing the allergy.

[M95D]

Now we do understand why allergies appear, or at least we have theories that are being tested with studies such as this one. It's all about chance, very literally.

As lymphocytes are formed, they randomly rearrange their T-cell receptor / immunoglobulin genes, creating a random antigen specificity for each cell. [1]

Then, they get selectively killed if they react to self-antigens. [2]

Those that survive, if they ever meet their specific antigen, will selectively multiply [3] and do random mutations again [4].

  [1] https://en.wikipedia.org/wiki/V(D)J_recombination
  [2] https://en.wikipedia.org/wiki/Clonal_deletion
  [3] https://en.wikipedia.org/wiki/Clonal_selection
  [4] https://en.wikipedia.org/wiki/Somatic_hypermutation

The current theory is that allergies appear if: (1) some random lymphocyte rearrangement created affinity for that allergen and (2) the allergen was not "known to be safe" by the selection mechanisms of the body and that lymphocyte was allowed to survive.

[themafia]

> Then, they get selectively killed if they react to self-antigens. [2]

There's also the ones that react weakly to self antigens and are kept around to put a check on everything else.

https://en.wikipedia.org/wiki/Regulatory_T_cell

The thymus is just insane.

[tialaramex]

Well, Mother Nature doesn't have to make any sense, so that's an advantage to her. This does make our nature - to try to understand - extremely difficult but makes no difference to her either way.

[robertjpayne]

Should clarify that the "hygiene hypothesis" has data supporting it for bacteria and allergens but not viruses.

[freedomben]

Always a good reminder that absolutism is dangerous. The medical field especially seems subject to this. It took me an insane number of years and multiple doctors and nearly being disabled from it to discover my thyroid condition in large part because "statistically it's mostly women who have thyroid issues" and the monstrously large range of "normal levels of TSH"

[tialaramex]

Huh. I'm male, my Thyroid is shot, and they weren't even surprised. They ran tests, they saw TSH is high and there's barely enough Thyroxine produced, and they went OK, you need Thyroxine, here's infinite auto-renewals, take as directed, and please get re-tested once a year to see if the dose needs changing.