The “hygiene hypothesis” proffers that infants and toddlers exposed to basic bacteria and dust/dander/pollen in their home environment 
may end up with a much healthier immune system that lasts for most of their life. If they are born on a farm and spend time outside among the crops and animals when they are young, they may fare much better than city kids living in a more sterile environment, the story goes. By being exposed to the plethora of microbes in dirt, plants, animals, etc, the immune system gets “trained” on how to respond to myriad antigens, and remembers those specific microbes for years and years.
Now, a new study in the medical journal Pediatrics posits that babies exposed to dogs may also achieve that same kind of “immune training” benefit. ”The researchers speculated that the pets may help boost immune system development. Children who live with dogs had fewer recorded ear infections and received fewer courses of antibiotics for ear infections, according to the team’s results.
“Contact with dogs protects children from respiratory tract infections during the first year of life…”
Wow. But apparently, it was the dogs only that had that kind of impact. “…The benefits seemed only to be linked to dogs after the results were adjusted for gender, rural living, maternal smoking, number of siblings, parental allergies and season of birth. The team concluded that the association for overall health with cat ownership was not significant.” And, six or fewer hours of exposure to dogs produced a benefit. Longer than six hours actually resulted in more sickness.
Of course, if I were writing the report, I would have said exposure may “balance” immune response rather than boost. Indeed, if the immune system is being trained to back off from an aggressive response to pet dander or saliva and let harmless microbes pass without needlessly creating inflammatory symptoms from immune overreaction, that is balancing in action, not boosting.
Just another tidbit in the vast world of how the immune system can balance itself with certain lifestyle factors in place.
