Thymus
stub — Seeded as a structural anchor for the immune-system; not yet written out.
The thymus is the primary lymphoid organ where T-cells mature. Thymic involution — the progressive replacement of functional thymic epithelial tissue with fat, beginning in early adulthood — is one of the earliest and most dramatic organ-aging processes and a central mechanistic driver of immunosenescence: as naïve T-cell output falls, the adaptive repertoire narrows, impairing responses to new antigens and vaccines. Thymic regeneration is an active rejuvenation target.