Primary lymphoid organs

The thymus is a flat, bilobed organ that sits behind the sternum, above the heart. Each lobe is subdivided into many lobules, separated from one another by connective-tissue septa, and each lobule itself has two zones: a densely packed outer cortex crowded with immature T cells (thymocytes), and a more sparsely populated inner medulla. The thymus secretes the hormone thymosin, which drives thymocytes to mature into fully immunocompetent T cells. The organ is most active during the neonatal and pre-adolescent years; by the early teens it begins to atrophy and is gradually replaced by adipose (fatty) tissue, which is why T-cell output declines with age. …