thymus

Organ in vertebrates, situated in the upper chest cavity in humans. The thymus processes lymphocyte cells to produce T-lymphocytes (T denotes ‘thymus-derived’), which are responsible for binding to specific invading organisms and killing them or rendering them harmless.

The thymus reaches full size at puberty, and shrinks thereafter; the stock of T-lymphocytes is built up early in life, so this function diminishes in adults, but the thymus continues to function as an endocrine gland, producing the hormone thymosin, which stimulates the activity of the T-lymphocytes.