Thymosin is a 5-Da polypeptide hormone secreted by the thymus gland, which stimulates the development of disease-fighting T cells (a specific type of white blood cell): The thymus gland is located behind your breastbone and between your lungs. Throughout your childhood, white blood cells called lymphocytes pass through the thymus, where they are transformed into T cells. Once the T cells have fully matured in the thymus, they migrate to the lymph nodes (groups of cells in the system immune system) throughout the body, where they help the immune system in the fight against disease. Although the thymus gland is only active until puberty (after puberty the thymus begins to slowly shrink and be replaced by fat), its dual function as an endocrine and lymphatic gland plays an important role in your long-term health. The body uses T cells to help destroy infected or cancerous cells. The T cells created by the thymus also help other organs of the immune system to develop properly. Thymosin also helps in the development of B cells to plasma cells to produce antibodies.