The thymus gland plays an important role in the body's immune system by helping develop T-lymphocytes, a type of white blood cell that protects the body through cellular immunity. Located in the upper chest, the thymus produces thymosin hormones that help T-cells mature and differentiate so they can recognize foreign cells and mount an immune response against infections and diseases. Though its size decreases with age, the thymus continues producing T-cells throughout life and remains an essential part of the immune system.