Once the cells and organs of the immune system have established "adult" levels or function, few studies ascertain maturation of these compartments beyond the formative years (Kincade et al. 2002). Several studies indicate that adult patterns of lymphopoiesis are remarkably stable even into the eighth decade of life (Hao et al. 2001; Nunez et al. 1996; Rossi et al. 2003). A notable exception is the thymus, which gradually involutes after puberty (Kendall 1991). Few new T cells are exported after the thymic cortex fills with adipose tissue, and immunity to new antigens depends on cross-reactions with previously encountered antigens (Nossal 2003). The thymus plays an important role in the development of sex organs, and infants with little or no thymus at birth (e.g., ataxia telangectesia and DiGeorge and cri du chat syndromes) fail to develop normal gonads (Kendall 1991). Although steroid hormones have been called "the gatekeepers" of entry into and progression within lymphoid lineages (Kincade et al. 2002) and obviously impact thymic development (Kendall 1991), few studies do more than state that final maturation of the immune system occurs during adolescence.
In general, aging beyond adolescence is associated with a reduced number of B cell progenitors in the bone marrow and a reduced proliferative capacity of the T cells. Surprisingly, the number of DC and NK cells increase in the periphery of older individuals, and DC seem unimpaired in their antigen-presenting functions (Longo 2003; Nossal 2003). The elderly seem more susceptible to infections, yet at the same time, they benefit significantly from immunization against influenza and pneumococci (Longo 2003; Nossal 2003). Knowledge of the relative senescence of various immune system components can aid in clinical manipulation to effect better health in older individuals.
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