During the first six months of life in rodents, there is enormous production of T and B lymphocytes from primary lymphoid tissues (Kincade 1981). Exposure to specific antigens during this period results in a rapidly expanding accumulation of lymphocyte specificities in the pool of memory B and T cells in secondary lymphoid tissues (Brooks and Feldbush 1983). As thymic function wanes at sexual maturity and thymocytes are no longer produced in that tissue, it is this pool of memory B and T cells that maintains immunocompetence for the life of the individual (Maki-nodan 1980). There is a similar, but less dramatic decline in the production of B lymphocytes with advancing age. This loss of B cell function serves to increase the dependence of older animals on memory cell function to maintain effective immu-nocompetence (Miller and Allman 2003).
Senescence of the immune system is well documented but poorly understood. Both innate and acquired immune responses to antigens are different in rodents in the last quartile of their life (McGlauchlen and Vogel 2003; Romanyukha and Yashin 2003). This failure of immune function is due, in part, to failure of production of newly formed cells and decreased survival of long-lived cells in lymphoid tissues.
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