The thymus: clock for immunologic aging?

A brief overview of the effects of age on T cell function is presented. Normal immune functions can begin to decline shortly after an individual reaches sexual maturity. Foremost among the cellular changes are those in the stem cells as reflected in their growth properties and the availability of precursor T cells, and in the T cells, in which a shift in subpopulations may be occurring. Present evidence indicates that thymic involution precedes, and therefore may be responsible for, the age-dependent decline in the ability of the immune system to generate functional T cells. It now appears that the primary effect of thymic involution is on a T cell differentiation pathway; the more mature T cells are affected first, the less mature T cells only later. Thus, the thymus may be the aging clock for the immune system. Current studies are centered on processes regulating growth and atrophy of the thymus, and methods for restoring the impaired immune function of elderly individuals.