Which structure gradually becomes smaller with increasing age?

$T$-cells are lymphocytes which produce cellular immunity. These are developed from:

According to the modern theory of aging,our body gradually loses its ability to resist bacteria and pathogens. This process is caused by the gradual disappearance (atrophy) of which organ?

What would happen to the immune system if the thymus gland is removed from the body of a person?

Damage to the thymus in a child may lead to:

Which of the following is $NOT$ associated with the thymus gland?