Naïve lymphocytes in young human adults have an enormously diverse antigen-recognition repertoire. A gradual decline in the production of naïve lymphocytes after puberty and a decrease in the diversity of the antigen-recognition repertoire occur during aging (Franceschi and Cossarizza, 1995)(Goronzy and Weyand, 2005). An immunosenescent state in the elderly is defined by a markedly decreased population of naïve lymphocytes and an oligoclonally expanded population of memory lymphocytes along with a reduced antigen recognition (Goronzy and Weyand, 2005).

Individual lymphocytes of various ages exist at any stage of life as a reflection of the generation of newly matured T cells, as well as repeated stimulation and proliferation of existing T cells. For human CD8⁺ T cells, young cells express CD28, whereas aged cells lose expression of CD28 along with multiple changes in gene expression. Gain and loss of expression of selected genes in young to aged cells is shown. Genes are grouped based on the physical location from cell surface to cytosol to nucleus. In addition to changes in gene expression, there is a decrease in the length of telomeres at the ends of chromosomes from young cells to aged cells as a result of cell divisions. Adapted from (Fann et al., 2005).