Re: Is there more growth of our Immune System then expansion of the Immunologic

In general the immune system does not improve with age and adult mammals rely on their lymphocyte memory to give them an edge in dealing with infectious microorganisms. T and B cells comprise the heart of this "insurance policy" and although there is contention on how long they last, memory T and B cells have been shown to persist as long as 2-3 decades and possibly longer (up to lifetime) in humans. If one considers age-dependent thymic involution, the case for decreasing T cell function becomes even stronger. The thymus is responsible for creating a pool of immature T cells (from stem cell precursors) that can best respond to infection, and its size decreases gradually with age.

Also, there is limited space for specific T and B cells in lymphoid organs such spleen, lymph nodes, and bone marrow. With age, immature T and B cells comprise a smaller portion of this pool as we accumulate antigen-experienced memory cells that can best deal with the same antigen should it show up again in the context of an infection. This in effect reduces the pool of "keys" that may best fit any new "locks" that turn up. Finally, there are reports in both humans and mice that virus-specific killer T cells (eg. recognizing influenza or HIV) from aged individuals are less effective in killing viral infected cells than those from young adults.

Hence in general it is better to encounter new pathogens sooner than later, ideally as children. Considering the popularity of antibiotic-laden cleansing products and the counter- productive intention of some parents to raise their kids in near-sterile environments, the adage that "if it doesn't kill you, it'll make you stronger" would seem to also apply to the development of the immune system. It is good for kids to get exposed to a variety of bacteria and viruses early on and avoid casual use of antibiotics, so that they can minimize allergies and maintain a well- stocked arsenal of memory lymphocytes that will serve them faithfully in old age.