Old people have weak immune system. What could be the reasons?

Explanation

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.
Thymus is degenerated in old individuals, resulting in decreased production of thymosin. As result the immune system becomes weak. in old people.