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
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.
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.
See it in 3D
The structures of Durgical's 3D anatomy where it is found.
Related
- Primary and secondary lymphoid organs
- CD4⁺ helper T cell
- CD8⁺ cytotoxic T cell
- Regulatory T cell (Treg, FOXP3)
- T-cell development: positive and negative selection
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 (OpenStax, Rice University (Anatomy and Physiology 2e), 2022); 21.3 The Adaptive Immune Response: T lymphocytes and Their Functional Types (OpenStax, Rice University (Anatomy and Physiology 2e), 2022); 22q11.2 deletion syndrome (MedlinePlus Genetics, U.S. National Library of Medicine, 2024).
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Representative model, for educational purposes only.