The thymus is a glandular organ which is involved in the establishment of the immune system from the 12th week of gestation until puberty. The thymus gland serves as the site of T cells (lymphocytes) maturation. Hematopoietic T lymphocyte progenitor cells produced in the bone marrow flow in the blood stream into the thymus where they are turned into small lymphocytes, called thymocytes, by thymic stem cells. Then, within this special thymic environment, these dividing lymphocytes go through a process of cellular differentiation in which immature thymocytes make distinct T cell receptors by a process of gene rearrangement.
The thymus is situated behind the sternum, in the upper antierior portion of the chest cavity, above the treachea. It is soft and pinkish-gray in color and is composed mainly of lymphatic tissue. At birth it is 5 cm long, 4 cm wide, and 0.6 cm thick. The thymus grows gradually in size and becomes more active until puberty; from then on, it slowly becomes atrophied and vestigial.