The aging immune system runs out of space, it's capacity taken up by memory T cells uselessly dedicated to cytomegalovirus. One approach is to remove all those errantly specialized memory cells (or eliminate cytomegalovirus early on). Another approach is to expand immune capacity by preventing or reversing involution of the thymus: "The immune system undergoes dramatic changes with age - the thymus involutes, particularly from puberty, with the gradual loss of newly produced naive T cells resulting in a restricted T cell receptor repertoire, skewed towards memory cells. Coupled with a similar, though less dramatic age-linked decline in bone marrow function, this translates to a reduction in immune responsiveness ... Given that long-term reconstitution of the immune system is dependent on the bi-directional interplay between primary lymphoid organ stromal cells and the progenitors whose downstream differentiation they direct, regeneration of the thymus is fundamental to developing new strategies for the clinical management of many major diseases of immunological origin."