Originally Answered: Would a human lose it's immune system if born into a sterile environment?
No, the immune system would still be there, and would work.
Here's the deal, though:
A new baby still has some immunity from its mother, and more from mother's milk. So it can slip in easily, and get introduced to new diseases gradually.
Also, when a child is growing, the cells are dividing rapidly and the immune system is very active. Later, after the person is grown, the immune system slows down too, so it takes a little longer to develop a new immunity. (In the general course of things, the person would already be immune to most things by the time the system slows down.)
How the immune system works: The body recognizes foreign proteins, and reacts against them. But to recognize that protein, the body has to have been introduced to it at some time. The first time can be rough.
That's the purpose of immunizations: They introduce the immune system to the proteins of some really nasty things, in a safe way. After that the body can react properly to the proteins of, for example, mumps or measles, without having to have first had the disease.