People who grow up on dairy farms only rarely develop asthma or allergies. This is probably because, as children, they breathe air containing bacterial components, which reduce the overall reactivity of the immune system. Schuijs et al. chronically exposed mice to bacterial endotoxin before the mice received an allergic stimulus. The protocol protected them from developing an allergic response. Protection relied on a particular enzyme: A20. In humans, a variant of A20 correlates with increased susceptibility to asthma and allergy in children growing up on farms.
M. J. Schuijs, M. A. Willart, K. Vergote, D. Gras, K. Deswarte, M. J. Ege, F. Branco Madeira, R. Beyaert, G. van Loo, F. Bracher, E. von Mutius, P. Chanez, B. N. Lambrecht, H. Hammad, Farm dust and endotoxin protect against allergy through A20 induction in lung epithelial cells. Science 349, 1106–1110 (2015). [Abstract] [Full Text]