Spleen
stub — Seeded as a structural anchor for the immune-system (with hematopoietic-system overlap); not yet written out.
The spleen is the largest secondary lymphoid organ: it filters senescent erythrocytes (red pulp), mounts blood-borne immune responses (white pulp), and serves as a reservoir for monocytes. Splenic aging contributes to immunosenescence through altered marginal-zone B-cell function and disrupted lymphoid architecture, and the spleen is a key site of extramedullary hematopoiesis under stress.