An overview of immune aging.

A brief overview of the effect of age on the function of cells of the immune system 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 cell where 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 affecting the more mature T cells first and only later the less mature T cells. Thus, the thymus may be the aging clock for the immune system. Future studies should be centered around processes regulating growth and atrophy of the thymus.