"Mice have a jungle of bacteria, viruses and fungi in their stomachs—and so do we.
These microorganisms help both mice and us break down dinner.
As we are finding, these bugs also help to regulate the immune system.
But we are just starting to learn how these tiny organisms influence us and how changing their composition changes us...
Interestingly, though, the mice with these microbes did not: their immune systems remained underdeveloped.
Even when researchers gave rat microbiota to mice, the mice's immune systems failed to mature"
http://www.scientificamerican.com/article.cfm?id=species-specific-microbes-immune