A small gland located in the upper chest cavity between the lungs and under the breastbone. It's a specialized lymphoid gland of the immune system and a component of the endocrine and lymphatic systems. The organ forms progenitor cells, which mature into thymus-derived cells (T-cells). The T-cells have a critical function in the immune system including helping other organs to develop appropriately and destroy invaders, such as infected or cancerous cells.