Thymus

• Is the bilobate encapsulated organ located behind sternum.
• Is the main organ for T cell maturation and
• It stays extremely active within fetal stage (max) and continuously atrophies at puberty.
• It is made up of cortical, medullary epithelial cells, stromal cells, interdigitating cells and macrophages.
• All cells are important in the differentiation of migrated T cell precursors and their screening before their movement to secondary lymphoid tissues.

The function of the thymus:

• Several hormones produced by the thymus stimulate the
• maturation of the T cells in the bloodstream where they identify and destroy pathogens,
• activate B cells to produce antibodies, and
• store the memory of past infections.

The most important lesson that T cells receive in the thymus as the central organ of the immune system is the competence to identify own cells (normal cells) from strangers. In the thymus, T cells learn to attack pathogens, infected cells, but not healthy cells! Only mature T cells are allowed to leave the thymus. This is one of the primary cause why our very powerful immune system does not attack our own body.

Source: Quara. Illustrations from Saladin, Anatomy & Physiology, 8th edition (McGraw-Hill).