Growing up on a farm improves the immunological responses that regulates allergies, autoimmune and inflammatory diseases. Although it isn't exactly clear what causes it:

> The researchers suggest additional work is required to determine the extent to which other farm-associated factors, such as social and maternal interactions, aerial contaminants, antigens from bedding and early nutrition, contributed to the impact of the environment on increased local and systemic immune regulation. [0]

[0]: https://www.sciencedaily.com/releases/2012/02/120208132549.h...