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Thymus is an encapsulated, bilobed gland located in the anterosuperior mediastinum that sits on the heart.
Derived from the third pharyngeal pouch ectoderm, the thymus consists of a dense cortex and pale medulla. The cortex contains immature T cells, while the inner medulla houses mature T cells and whorled calcifications of epithelial reticular cells called Hassall corpuscles—note that T cells move centrally as they mature within the thymus. It is important to note that the thymus normally involutes or atrophies with age. Young children present with large thymuses up until puberty, after which, it decreases in size.
Thymus is anatomically important, as it is the main site of differentiation for T cells, which are initially produced in the bone marrow. T cells are responsible for the cellular aspect of adaptive immunity as their various subsets (CD4+, CD8+) help differentiate B cell antibodies, activate macrophages, and can directly kill virally infected cells. As T cells move centrally from the cortex to the medulla, they are often presented with antigens via APCs (B cells, dendritic cells, macrophages). Presentation of antigens then allows for thymic selection (positive and negative selection) and fully mature and activated T cells ready for secretion and circulation to secondary lymphatic organs.