Hmm, I'm also wondering about studies about overly sanitized environments for children being correlated with higher allergy rates.

I guess poking around for a good representative study, it's actually low diversity of microbial exposure, not "cleaning" per-se that is correlated - e.g this is one reason why households with dogs have lower allergy rates. A monoculture of certain tree species also implies less microbial diversity.

I'd like to preemptively draw a line between two different kinds of hypothesis when it comes to hygiene:

1. The immune system is not being exposed enough to wild or even infectious content, and it needs more threats to fight off.

2. ("Old Friends") The immune system is not being exposed enough to commensal or even symbiotic organisms that we co-evolved with, throwing off its calibration and tuning.

I instinctively prefer the second, the first seems a little too simple, like some some scaled-down version of "tough love" and "spare the rod[-bacteria], spoil the child."

It can go the other direction, too: exposure to moldy home environments gave me (now resolved) food sensitivities, dust allergies, pet-associated allergies, etc.

You can definitely undertrain, or overwhelm, the immune system if not cautious!