Write in detail about the thymus.

(N/A) The thymus is a lobed organ located near the heart and beneath the breastbone (sternum).
The thymus is quite large at the time of birth,but it gradually reduces in size with age,and by the time puberty is attained,it reduces to a very small size.
Both the bone marrow and the thymus provide a microenvironment for the development and maturation of $T$-lymphocytes.
The stem cells of the bone marrow,which give rise to $T$-lymphocytes,must pass through the tissue of the thymus gland before they become fully functional.
In the thymus,they develop into cells called thymocytes. Some of the thymocytes mature into $T$-cells.
These $T$-cells leave the thymus gland and enter the bloodstream. Some remain in the blood,while others migrate to the tissue fluid,lymph nodes,and other organs such as the spleen.