CD28 and infection: One of the hallmarks of immune aging is a decrease in the proportion and number of peripheral initial T cells and an increase in the number of memory T cells (Table 1).28 This change has been identified as one of the main reasons for the increase in infection and cancer incidence in the elderly.29 Recent studies have shown that the accumulation of CD8+ memory T cells is mainly the result of persistent antigen stimulation caused by herpesvirus (especially cytomegalovirus) infection.30 Another sign of T cell senescence is the decreased expression of CD28 molecules.