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It is a specialized agency of the immune system in the thymus. In the adaptive immune system in athymic “education” T lymphocytes are important cells (T cells).
Each T cells to attack foreign material identification and its receptor. T cells have a receptor produced by shuffling the gene segments randomly. Each T cells, attack the different antigens. T cells that attack the protein of the body’s own is removed in the thymus. Thymic epithelial cells express large proteins from other parts of the body. T cells undergo “positive selection” is arranged that in contact to buried MHC expressed by thymic epithelial cells and does not affect the first cage. Secondly, (to prevent autoimmune diseases), T cells, those with intermediate affinity eliminated by apoptosis “negative selection” reacts with thymic dendritic cells by T cell interaction with high-affinity I receive the difference survive.

In the thymus, is composed of the same sheet of two, is located in the anterior superior mediastinum anatomically in front of the center of the chest and heart.
Histologically, the thymus is surrounded by capsule and outside the center, and can be divided in the brain cortex, the area. Medulla and cortex play a different role in the development of T cells. Thymocytes could be (obtained from local bone marrow hematopoietic stem cells) cells of hematopoietic origin and thymic stromal cells is divided. The development of T cells in the thymus, this is the hematopoietic origin. Stromal cells, including dendritic cells, thymic cortical epithelial cells, and medullary thymic epithelial cells.

It provides a T lymphocyte induction environment from hematopoietic stem cells in the thymus. In addition, thymic stromal cells makes it possible to select functional, self-tolerance of T-cell repertoire. Therefore, one of the most important role of the thymus is the induction of central tolerance. Thymus is the most active and largest during the period of pre-adolescence and newborn. At the beginning of puberty, the thymus starts the thymus atrophy, connective tissue, has been replaced by the (fat) tissue fat. However, residual T limfopoyezisno continues through adulthood.

The Galen, be noted that the size of the organ has changed over the period of human life was the first time. Since the number of apoptotic lymphocytes, the more thymus dismissed as “lymphocyte graveyard”, without functional significance initially. Similar to the old mice three days by surgical removal of the thymus, the importance of the thymus in the immune system, 1961 by Jack Miller than the deficit of the latest of lymphocyte population of T cells after the organ of origin of the subsequent I was discovered in years. In recent years, advances in immunology have allowed it is possible to understand more fully the function of the thymus in the maturation of T cells.

Two main components of thymic epithelial cells thymus, and lymph thymus cells, have the origin of the various development. Is the first to develop the thymic epithelium, two flask form mesoderm diverticula that occur on either side of the third branchial pouch (pharyngeal pouch), is displayed in the form of 1, the back and lateralward the neural tube and mesoderm surrounding The derived mesenchymal before expansion of the abdominal aorta.

Epithelial thymocytes and meets the connective tissue, I participate here. Pharyngeal opening of the duodenum are removed immediately, but the neck of the flask, and lasted for some time as a portable cable. The spread of cells, separated surrounded invade the mesoderm, bud cells are formed on the surface of the flask further. Additional piece of thymus tissue has been developed from time to time fourth gill bag. In the later stages of thymic epithelium, hematopoietic precursors of bone marrow, migrate to the thymus. Development of normal thymus is dependent on the interaction of hematopoietic thymocytes and thymic epithelium between then.

After continuing to grow during puberty the thymus, birth and led by high levels of circulating hormones, thymus begins to atrophy. Is proportional to the thymus size, activity of the thymus gland is the most active of (T-cell output) before puberty. When atrophy, body, replaced with (a phenomenon called “regression organ”) primarily fat and activities If the size has decreased dramatically. The atrophy due to circulating levels of chemical castration or physical adult thymus result of increased activity and size and sex hormones increased. That (70%) has a malignant tumor or thymic hyperplasia case patients, myasthenia gravis, many of autoimmune disease has been found. Order or reason for this situation has not been determined.