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Thymus Gland: Function, Structure and Diseases

The thymus gland is a central organ of the immune system, most active during childhood, responsible for the maturation of T-lymphocytes essential for immune defense.

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The thymus gland is a central organ of the immune system, most active during childhood, responsible for the maturation of T-lymphocytes essential for immune defense.

What Is the Thymus Gland?

The thymus gland (or simply the thymus) is a specialized organ of the immune system located behind the breastbone (sternum) in the front part of the chest cavity (anterior mediastinum). It belongs to the primary lymphatic organs and plays a critical role in the development and maturation of T-lymphocytes (T-cells), which are essential for the body's immune defense. The thymus is largest and most active in newborns and children; after puberty, it gradually shrinks and is replaced by fatty tissue in a process known as thymic involution.

Structure and Location

The thymus consists of two lobes (lobi thymi), each divided into smaller lobules. Every lobule has an outer cortex and an inner medulla:

Cortex: Immature T-cells (thymocytes) migrate here and undergo initial stages of maturation.
Medulla: This is where negative selection takes place, eliminating T-cells that might attack the body's own tissues. The medulla is also characterized by Hassall's corpuscles, which are spiral arrangements of epithelial cells.

Functions of the Thymus Gland

The primary function of the thymus is the maturation and selection of T-lymphocytes. This process occurs in several steps:

Positive selection: Only T-cells capable of recognizing the body's own MHC molecules (major histocompatibility complex) survive.
Negative selection: T-cells that react too strongly to the body's own structures are eliminated through apoptosis (programmed cell death) to prevent autoimmune reactions.
T-cell differentiation: Surviving T-cells mature into functional CD4+ helper T-cells or CD8+ cytotoxic T-cells and leave the thymus as naive T-lymphocytes.

In addition, the thymus produces hormones such as thymosin, thymulin, and thymopoietin, which support T-cell maturation even outside the thymus.

Development and Involution

The thymus develops embryonically from the third pharyngeal pouch and is already functional at birth. It reaches its maximum weight (approximately 30–40 g) around puberty. Afterward, involution begins: thymic tissue is progressively replaced by fatty tissue, and the production of new T-cells declines. In adults, the thymus is still present but significantly reduced in size and activity.

Diseases of the Thymus Gland

Several conditions can affect the thymus:

Thymoma: A tumor of the thymic epithelial cells; frequently associated with myasthenia gravis (an autoimmune neuromuscular disorder).
Thymic carcinoma: A rare, more aggressive malignant tumor of the thymus.
DiGeorge syndrome: A congenital defect in which the thymus is absent or underdeveloped, leading to severe T-cell deficiency and immunodeficiency.
Thymic hyperplasia: An enlargement of the thymus, which can occur in autoimmune diseases such as myasthenia gravis.

Clinical Significance

The thymus gland is essential for the development of a functional immune system. Dysfunction of the thymus can lead to immunodeficiency, increased susceptibility to infections, or autoimmune diseases. In certain cardiac surgeries performed on infants, partial removal of the thymus may be necessary; the long-term effects on the immune system are carefully considered. In oncology, the thymus is relevant to the development of T-cell lymphomas.

References

Janeway C.A. et al. - Immunobiology: The Immune System in Health and Disease. 9th edition. Garland Science, 2017.
Abbas A.K., Lichtman A.H., Pillai S. - Cellular and Molecular Immunology. 10th edition. Elsevier, 2021.
World Health Organization (WHO) - Classification of Tumours of the Thymus. IARC Press, Lyon, 2015.

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