|MadSci Network: Immunology|
There is a fair amount of study looking at the function of the immune system for mice bred and housed in "germ-free" environments. It is important to remember that in your intestines there is a considerable number of bacteria present of different varieties. So a truly "germ-free" person would be lacking those bacteria as well which would cause problems with our digestion (many of the things we eat require the aid of the bacteria in the gut to be digested). In germ-free mice it is clear that over time there is some degradation of the immune system in the sense that the number of peripheral blood mononuclear cells drop and antibody levels drop as well. However, these mice are still fully resistant to bacterial and viral pathogens. It is true that immunological memory can decline over time but that is true whether you are germ-free or not. No studies have been down (that I know of) to determine if the rate of decline is any different in germ-free animals. You cannot necessarily extrapolate what happens in the mouse to a human but we can compare humans from developed nations versus third-world nations. People from countries such as the United States have above- average health care, are vaccinated for a number of infectious diseases and have clean drinking water which effectively limits the number of pathogens we encounter in our lifetime. As a result the peripheral blood mononuclear cell number and antibody levels are significantly reduced compared to people from third-world nations where water is unclean and infectious diseases are rampant. People from the United States may require vaccination against native diseases when traveling out of the country but they are no more susceptible to these diseases than the native population. It is an important question, however, and one that I am sure the people at NASA are very keen to answer with the astronauts aboard the International Space Station - although they are not technically in a sterile environment.
Try the links in the MadSci Library for more information on Immunology.