Saturday, April 26, 2008

Thymus Gland

The thymus is a vascular organ of the lymphatic system situated just behind the breastbone. The human thymus continuous to grow for about a year after birth, reaching a weight of about 42 g; this size is maintained until puberty. After puberty the lymphatic tissue is replaced by fat, but the thymus remains functional throughout life.

The main function of thymus is to process lymphocytes received from the blood producing bone marrow and fetal liver. These cells proliferate and differentiate in the thymus into thymic lymphoid cells called T cells, each one programmed for the number of antigens to which it will react. In humans the cellular immune system requires the presence of the thymus at birth; this system allows the body to recognize foreign, that is "non self" tissue and to attack malignant cells, viral infection, fungal infection, and some bacteria. Little is known of the factors and processes of thymic function. The importance of the thymus to the human immune system, however, is readily demonstrated in some patients with congenital thymic deficiency states by the restoration of immunological responsiveness after fetal thymus graft.

The thymus gland is an organ of the lymphatic system, which protect the body against infection. Located behind the sternum, near the heath and lungs, it is well supplied with blood vessel. Its two main lobes are each subdivided into numerous lobules; a network of delicate connective tissue holds the lobes together. Within each lobule are two zones of tissue, inner zones called the cortex and an outer zone called the medulla. The cortex is composed of lymphocytes, while blood cells that produced antibodies and attack bacteria; this lymphocytes are packed into a fiber structure called a reticulum. The medulla has a more cellular reticulum and contains thymic corpuscles, which are concentric clusters of epithelial cells enclosing a core of granular cells. The function of these corpuscles is not yet understood. The thymus is most active during fetal and childhood growth. Its main function appearing to be the production of lymphocytes and the destruction of defective lymphocytes. The thymus may also secrete a hormone that influence the response of lymphocytes for foreign tissue. After puberty the thymus slowly degenerates.

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