[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Immunology](https://durgical.com/en/immunology/) › [Lymphoid organs](https://durgical.com/en/immunology/#a-organs) › Thymus: where T cells learn [Immunology](https://durgical.com/en/immunology/) · [Lymphoid organs](https://durgical.com/en/immunology/#a-organs) # Thymus: where T cells learn - Organ The thymus, behind the sternum, is where young T cells are tested: those that cannot see self-MHC or that attack self are removed. Updated 2026-11-01 **See it in 3D:** [Thymus](https://durgical.com/en/anatomy/thymus/) ## Key points - Cortex: dense, dark-staining, full of immature thymocytes; positive selection (binding self-MHC) happens here. - Medulla: paler; negative selection removes T cells that bind self-antigen strongly; Hassall corpuscles are found here. - Only a small fraction of the thymocytes that enter leave as mature T cells. - DiGeorge (22q11.2 deletion): absent or small thymus, low T cells, low calcium from missing parathyroids, heart defects. - An absent thymic shadow on a newborn chest X-ray suggests DiGeorge syndrome or severe combined immunodeficiency. ## What it is The thymus is a two-lobed primary lymphoid organ in the anterior mediastinum, between the sternum and the great vessels above the heart. Immature T cells arrive from the bone marrow and are selected: only those whose receptor recognizes self-MHC weakly survive, and those that react strongly to self-antigens die. It is largest in childhood and shrinks (involutes) after puberty, but the T cells it already made persist. ## Structure - Capsule and septa divide each lobe into lobules; each lobule has an outer cortex and an inner medulla. - Epithelial reticular cells form the framework and present self-peptides to developing T cells. - Thymic epithelium develops from the third pharyngeal pouch. - After puberty functional tissue is gradually replaced by fat (involution). ## Step by step - T-cell precursors leave the bone marrow and enter the thymic cortex. - They rearrange their T-cell receptor genes and express both CD4 and CD8 (double positive). - Positive selection in the cortex: cells that bind self-MHC survive; the rest die by neglect. - They keep CD4 if they bound MHC class II, or CD8 if they bound MHC class I. - Negative selection deletes cells that bind self-antigen too strongly; it starts in the cortex and continues in the medulla, where AIRE-driven tissue antigens are shown. - Mature single-positive naive T cells leave through the medulla to the blood. ## Clinical notes - DiGeorge syndrome: recurrent viral, fungal and opportunistic infections, neonatal tetany from hypocalcemia, conotruncal heart defects. - Thymoma (a tumor of thymic epithelium) is linked to myasthenia gravis. - Failure of negative selection contributes to autoimmunity; some regulatory T cells are also made in the thymus. - Anterior mediastinal masses in exams: thymoma, teratoma, lymphoma and retrosternal thyroid. ## Memory aid The thymus is a strict exam with two questions: first, can you see MHC at all? (cortex). Second, do you attack your own house? (medulla). Pass the first, fail the second. ## Test yourself Think of the answer, then tap the question to see it. ## Where in the thymus does positive selection happen, and what does it test? In the cortex; it keeps T cells whose receptor can bind self-MHC. ## What does negative selection remove? T cells that bind self-antigens too strongly, which could cause autoimmunity. ## Which immune, endocrine and heart findings define DiGeorge syndrome? Small or absent thymus with low T cells, hypocalcemia from missing parathyroid glands, and conotruncal heart defects. ## What happens to the thymus after puberty? It involutes: lymphoid tissue is gradually replaced by fat, though existing T cells persist. [Add to my cards (4)](https://durgical.app/estudiar/?add=/en/immunology/thymus-where-t-cells-learn/) ## See it in 3D The structures of Durgical's 3D anatomy where it is found. - [Thymus](https://durgical.com/en/anatomy/thymus/) ## Related - [Primary and secondary lymphoid organs](https://durgical.com/en/immunology/primary-and-secondary-lymphoid-organs/) - [CD4⁺ helper T cell](https://durgical.com/en/immunology/cd4-helper-t-cell/) - [CD8⁺ cytotoxic T cell](https://durgical.com/en/immunology/cd8-cytotoxic-t-cell/) - [Regulatory T cell (Treg, FOXP3)](https://durgical.com/en/immunology/regulatory-t-cell/) - [T-cell development: positive and negative selection](https://durgical.com/en/immunology/t-cell-development-positive-and-negative-selection/) [← All of immunology](https://durgical.com/en/immunology/) Educational content on the immune system; not a treatment or individual vaccination guide and no substitute for a health professional's advice. No doses are given. Sources: [21.1 Anatomy of the Lymphatic and Immune Systems](https://openstax.org/books/anatomy-and-physiology-2e/pages/21-1-anatomy-of-the-lymphatic-and-immune-systems) (OpenStax, Rice University (Anatomy and Physiology 2e), 2022); [21.3 The Adaptive Immune Response: T lymphocytes and Their Functional Types](https://openstax.org/books/anatomy-and-physiology-2e/pages/21-3-the-adaptive-immune-response-t-lymphocytes-and-their-functional-types) (OpenStax, Rice University (Anatomy and Physiology 2e), 2022); [22q11.2 deletion syndrome](https://medlineplus.gov/genetics/condition/22q112-deletion-syndrome/) (MedlinePlus Genetics, U.S. National Library of Medicine, 2024). ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake Representative model, for educational purposes only. --- Source: https://durgical.com/en/immunology/thymus-where-t-cells-learn/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/immunology/thymus-where-t-cells-learn/