Timing is everything...

...in the life of the lymphocyte.
Originating in the bone marrow, a subset of lymphocytes migrates during gestation to the thymus gland. There they "learn" to identify antigens, both foreign and native. It is in childhood that these cells are most active, when they set their "memories". In later life they are then prepared for the daily onslaught of parasites, toxins, and foreign bodies that seek to harm us.

Research has shown that children who are "unexposed" to a broad array of antigens have difficulties later with immune disorders of all types. The thymus involutes by puberty, so this timing makes perfect sense. Parents who misguidedly "protect" their children from common antigens and vaccines are doing them no favors. Our immune systems have evolved to use childhood as the time during which we lay down a healthy and vigorous immune system--there is no data to suggest it can be "overwhelmed" except by real disease at this stage. The fact that we are seeing a generation of immune deficient, hypersensitive, and disordered young people is a direct result of our scientifically illiterate approach to our own bodies and those of our children.
In a very real sense, children who are allowed to get "dirty" are healthier than those who are kept "too clean." Recent studies document this as a direct cause of the increased incidence of allergic asthma specifically.
And children seem to know this....