The size of this organ is large at the time of birth,but it gradually shrinks as age increases.

The thymus gland is a lobular structure located on the $A$ side of the aorta and plays a significant role in the development of the $B$ system.
Choose the correct combination of $A$ and $B$.

Which tissue is large at birth and gradually shrinks as one ages?

Damage of thymus in a child may lead to

Which of the following gland grows to the maximum size at puberty and then diminishes gradually?

Write in detail about the thymus.