Rule 1: The Child Who Eats the Most Dirt Wins
Dismayed first-time parents often watch their crawling youngsters roll through life like little vacuum cleaners, sampling every piece of debris they can find. Much of this behavior is teething-related, but its other, more critical role is to help stimulate development of the naive immune system.
Immune development starts in the thymus, a large gland located between the heart and the sternum of the child where “T” cells are produced. (This gland shrinks and largely disappears by puberty, which is why early immune training is so critical.) The first thing young T-cells learn is to differentiate between those tissues that belong to the body and those that don’t.
It’s as if the T-cells are learning to identify the uniform of the home team. The harmless organisms that find their way into the child’s system through those taste-testing forays provide recognizable examples of “away” team uniforms, thereby helping the system learn which cells to protect and which to vanquish.
This “self/nonself” discrimination is the basis for a well-trained immune system, one that does not attack its own cells. The absence of this ability is “autoimmune” disease, or “self-immunity,” where the body goes after its own tissues.
Upon recognition of an invader, T-cells multiply wildly. If all goes well, the invader is wiped out and the immune guardians go back to their original surveillance role, a little wiser for the next invasion.
Since every exposure provokes the multiplication of T-cells, the more frequent the exposure, the more powerful the standing immunological army, and the larger our immunological reserve. Over a lifetime of this challenge-and-response dynamic, a healthy person’s immune system grows so well-trained that it can knock out invasion attempts before a single symptom becomes noticeable.