 As a person ages, their immune system undergoes a series of changes, in a process referred to as immunosenescence.Notable among these changes is a decreased production of naive T cells and naive B cells.The reduced number of naive lymphocytes (T and B cells) is attributed to the fact that the telomeres in hematopoietic stem cells (HSCs), degenerate over time and, consequently, limit the proliferation of HSCs and production of lymphoid progenitor cells.