What the programme was mentioning is that children (as well as adults) these days spend a lot less time outside than when we used to, and that the good bacteria in our guts doesn't get a chance to get there - which research seems to link, having a decent amount of bacteria in your gut tends to lead to less allergies. By exposure to less clinical areas is how we develop immunity. In fact we start to build this immunity as soon as we are born, where when still within the amniotic fluid we are in a pretty much sterile environment, and by having a normal birth (non Caesarian) it is believed that the baby is exposed to their mother’s bacteria (on the way out), whereas a c-section babies are left vulnerable by avoiding the journey through the birth canal since they won't have that exposure to the bacteria on the way out. Apparently c-sections can increase a child's chance of developing allergies fivefold.