Naive vs. memory T cells

Naive T cells are antigen-inexperienced lymphocytes that continuously recirculate through secondary lymphoid organs awaiting their cognate antigen; they are generated in the thymus and depend on tonic TCR and IL-7 signals for peripheral survival. Upon antigen encounter and co-stimulation, they clonally expand and differentiate into effector cells and, subsequently, long-lived memory T cells (central, effector, and tissue-resident subtypes) that enable faster, stronger recall responses. With age, the naive compartment contracts due to thymic involution and homeostatic proliferation-driven phenotypic conversion, while the memory compartment expands — particularly oligoclonal CD8+ populations driven by persistent viral antigens such as CMV. This shift narrows the T-cell receptor repertoire and limits the capacity to respond to novel pathogens or vaccines.