Figure 1. Compared with an elderly, the absolute number of naïve T-cells is higher with diverse T-cell receptor repertoire in a young adult. When antigen is presented by antigen presenting cells, effector T-cells and memory T-cells are quickly produced. In an elderly, the absolute number of naïve T-cells are decreased with limited T-cell receptor repertoire. Chronically stimulated effector T-cells and memory T-cells predominate with progressive expansion of oligoclonal T-cells.