The pyramidal organ located above the heart between the trachea and sternum is called the thymus. It is soft and bifurcated. Children’s thymuses are large and active. The thymus secretes the hormones thymosin and thymopoietin. Hormones regulate lymphocyte maturation. The thymus gradually turns into fat tissue during puberty. But it disappears with age. White blood cells produced in the red bone marrow reach the thymus and become T-lymphocytes. These are any immune cells that turn into T-effector, T-killer and T-helper cells and recognize viruses and bacteria.