A 2002 study published in JAMA showed that children exposed to two or more pets during the first year were 66 to 77 percent less likely to have "allergic sensitization to multiple allergens during childhood." Why? It has to do with your gastrointestinal microbiome, or the bacteria inside your gut. In a 2013 study, researchers introduced dust from a house with a canine to lab mice. When these mice came into contact with cockroach or egg allergens, their asthma symptoms were significantly reduced. This is largely to do with a bacteria called Lactobacillus johnsonii that was found in the microbiome of the protected mice, ostensibly because of that dog dust. The same thing may be true when your little ones are born into a household with pets.
The growing body of evidence backing early exposure to allergens relates to the hygiene hypothesis, or the theory that the more germs we're exposed to, the better we can resist them. Routine exposure to two or more pets during the first few years of life has the most dramatic impact on a child's immune system. So how old should your kiddo be exposed? Doctors say, the younger the better. Johns Hopkins says that the protective effects of germs, pet dander, and insects disappear after baby's first year.