Thymosin is a hormone secreted by thymus gland which play a major important role in development of immune system, but with the increasing age thymus gland degenerated or in the old people thymus gland starts to degenerate and thus thymosis hormone produce less, hence the old people have weak immune system.Thymosin is responsible for the differentiation maturation and development of T-lymphocytes and T-cells which provides cell- mediated immunity. So thymosin is the important hormone that play an important role in immune system but with age thymus gland starts to degenerate and thus old people have weak immune system.