The thymus, also called the thymus gland, is a small organ behind the breastbone that most people first hear about during an ultrasound. Its name comes from its shape: in a child it resembles a two-pronged fork or a thyme leaf (Greek thýmos). Functionally it is a central organ of the immune system, the place where T lymphocytes learn to tell self from non-self. Below is where it sits, how it works, which peptides it makes and why little more than fat tissue remains of it later in life.

Where the thymus gland sits

The thymus lies in the anterior mediastinum, directly behind the sternum, above the heart and the great vessels. It has two lobes under a capsule, and within each lobe anatomists distinguish a cortex and a medulla. In a newborn the organ weighs roughly 15 g, grows to about 30–40 g by adolescence, then begins to shrink. In a middle-aged adult it is often hard to make out on a scan, because active tissue has been replaced by fat.

What the thymus does: a school for T cells

T cell precursors form in the bone marrow and arrive in the thymus immature. There they go through a selection that immunologists describe in two stages.

  • Positive selection. In the cortex a cell must show that its receptor can recognise major histocompatibility complex molecules. Cells that cannot, die.
  • Negative selection. In the medulla the cell is shown the body's own proteins. If it reacts too strongly, it is removed. This is how tolerance to self tissue is built.

Selection is harsh: only a few percent of the cells that enter the thymus leave it. Those that pass enter the blood as naive T lymphocytes and form the repertoire the body will use against new infections. Palmer's review summarises how this process changes with age (Frontiers in Immunology, 2013).

Thymic hormones

Beyond selecting cells, the thymic epithelium produces its own signalling peptides. The best known are:

  • Thymosin α-1, a 28-amino-acid peptide and the best characterised member of the group.
  • Thymopoietin, linked to T cell differentiation.
  • Thymulin, a zinc-dependent peptide whose activity falls with age.

These molecules gave rise to a whole class of drugs and research compounds. An independent 2024 review collects the properties of thymic peptides and how far their clinical application has actually got (Int. J. Peptide Res. Ther., 2024). We profiled the most studied of them separately: thymosin α-1.

Why the thymus shrinks with age

Thymic involution, the gradual replacement of active tissue by fat, starts early. It was long explained by puberty, but a morphometric study by Steinmann and colleagues showed that the thymic epithelium shrinks on its own schedule, independently of puberty (Scand. J. Immunol., 1985). A review by Chinn and colleagues describes how the bone marrow changes alongside it (Seminars in Immunology, 2012).

The practical consequence is simple: the less the thymus works, the fewer new naive T cells reach the blood. The body leans more on memory cells it already has, and its ability to answer an unfamiliar pathogen narrows.

Can the thymus be brought back to work

The question is open. Boehm and Swann treat involution and regeneration as two sides of one mechanism and list the approaches tried in animals (Nature Reviews Immunology, 2013). In humans there is still no reliable way to restore the organ, and what gets discussed sits at the level of small early studies. We covered that topic, together with the peptide line, separately: thymic involution and what has been tried.

When the thymus reaches a doctor

In children the organ is examined by ultrasound when thymomegaly, an enlarged gland, is suspected. In adults the thymus usually comes up in a different context: thymomas, meaning tumours of the gland, and myasthenia gravis, an autoimmune disorder of neuromuscular transmission directly linked to the thymus. These are diagnoses, not a subject for conclusions drawn on your own.

Longeva supplies thymic peptides as reagents for research and educational use. They are not medicines and are not intended to treat or prevent disease. Questions about your own health belong with a doctor.

Read also: Types of immunity and immune peptides.