A major role in the development of the immune system is played by thymus.
The thymus gland is a lobular structure located on the dorsal side of the heart and the aorta. It is derived from the endoderm of the embryo. Thymus secretes a hormone named thymosin which stimulates the development of white blood cells(WBCs), involved in producing immunity. In old individuals, thymus is degenerated which results in decreased production of thymosin. The immune system as a result becomes weak, in old people.