CD28 and lymphopenia: Of note, markers of immune aging have been documented in the ADs in humans [69,88,89,90,91,92,93,94,95,96] (Table 1), such as reduced percentage of TREC+ cells and/or lymphopenia, LIP in T cell subsets, and loss of CD28+ cells/accumulation of CD28null cells in T cell subsets (Table 1).