Thymus

Dr. Manuela Mai studied medicine at the Universities of Heidelberg and Mannheim. After graduating, she gained clinical experience in gynecology, pathology and clinical pharmacology. She is particularly interested in the broader connections that lead to diseases - also outside of conventional medicine. She completed additional training in classical homeopathy as well as ear and skull acupuncture.

More about the experts All content is checked by medical journalists.

The thymus (thymus gland, sweetbreads) is an important part of the immune system. In the thymus, certain white blood cells (lymphocytes) get their immunological character, at the same time the maturation of disease-fighting T cells is stimulated by thymus hormones. The thymus is only active in childhood and adolescence. Read everything about the thymus and its function as a "school of the body police".

What is the thymus?

The thymus plays an important role in the human immune system. In this small organ, some of the white blood cells (T lymphocytes or T cells) learn to recognize and attack foreign cells. To do this, the immune cells are shaped in such a way that they can distinguish the body's own surface structures (antigens) from, for example, bacteria or viruses from exogenous antigens. This is important to prevent the immune cells from attacking your own body and so-called autoimmune diseases from developing.

The thymus consists of a right and a left lobe, both of which are surrounded by a connective tissue capsule. From this capsule, strands of connective tissue pull through the lobes and divide the thymus into many small lobules called the lobules thymi. Each lobule consists of a light marrow zone (medulla), surrounded by a darker cortex (cortex).

The characteristic Hassall corpuscles are found in the medullary zone of the thymus gland. They are particularly easy to see under the microscope. The Hassall corpuscles probably consist of coalesced cover tissue cells (epithelial cells) and look like small onions due to this layering. Their function has not yet been clarified, but it is believed that they help the immune cells to mature.

Change of the thymus gland

The thymus is not equally active throughout life: the thymus begins its work with the production and maturation of T lymphocytes even before birth.

In the newborn, the thymus cicra is five inches long and two inches wide. In the course of childhood through to puberty, the thymus reaches its maximum weight of 35 to 50 grams. Function and tissue change. In old age you will mainly find fat and connective tissue, the weight is reduced to about three grams. This process is called thymus involution. Most of the development of the immune cells has already been completed.

After its regression, the secondary lymphatic organs (lymph nodes, spleen) take over the tasks of the thymus.

What is the function of the thymus?

The thymus, together with the bone marrow, is known as the primary lymphatic organ. This means: the immune system develops and matures in the thymus and bone marrow.

To do this, the immune cells go through several stations:

Bone marrow

“Multipotent stem cells” migrate from the bone marrow, they are precursor cells whose basic function has already been established, but development is not yet complete.

Thymus

These cells reach the thymus via the bloodstream. In order to maintain the imprint and differentiation, the precursor cells (thymocytes) have to migrate through the thymus from the cortex to the marrow region, in order to then be released back into the bloodstream as T lymphocytes.

Imprinting of the immune cells

Embossing takes place in three steps. Then those cells are sorted out that have not been “trained” properly or not well enough. Over 90 percent of the imprinted cells are eliminated.

At the end of the imprinting and selection, the remaining T-lymphocytes have learned to differentiate the body's own tissue from foreign tissue by recognizing the surface structures accordingly. You can later identify and attack bacteria, viruses, parasites or tumor cells, and the body's own cells are spared.

Relocation to the lymph nodes

After their "training", the T lymphocytes are released back into the blood and thus enter the lymph nodes. There they are waiting to be deployed. If a T-cell recognizes its very special surface molecule in the case of an intruder, this T-cell multiplies. Together, the clones attack bacteria, for example. This is how an infection is warded off.

Thymus: hormone production

Why is this organ also called the thymus gland? The function of the thymus as a gland is also the production of thymosin, thymopoietin I and II. These hormones play a role in the maturation and differentiation of the T lymphocytes in the thymus.

Where is the thymus located?

The thymus is located in the chest, directly in the middle behind the upper third of the breastbone (sternum). It is located above the pericardium in the middle membrane (mediastinum) and extends roughly from the attachment of the collarbones to the fourth pair of ribs.Due to its position over the pericardium, the thymus sits on the large vessels of the heart, the main artery (aorta) and the superior vena cava (superior vena cava). The thymus is laterally bounded by the pleura and the lungs.

What problems can the thymus cause?

Due to the complex structure of the thymus, abnormalities can occur more frequently. However, this does not necessarily mean that its function is impaired. If so, impairment plays a role especially at a young age when the thymus is active.

For example, there are congenital disorders in which the thymus does not develop at all (thymus aplasia) or is only partially developed. This developmental disorder can lead to pronounced immunodeficiencies with a high susceptibility to infections. Often thymus aplasia is also an accompanying symptom of other hereditary defects, such as DiGeorge syndrome, retinoid embryopathy, Louis-Bar syndrome or Wiskott-Aldrich syndrome.

Especially in early infancy it can happen that the thymus enlarges (persistent thymus hyperplasia) and presses on the trachea, causing breathing difficulties. Most of the time, however, it regresses spontaneously.

The thymus also seems to play a role in a certain severe autoimmunological disease of the skeletal muscles (myasthenia gravis pseudoparalytica) - in many patients the thymus is also enlarged here

An abscess can also form in the thymus. An abscess is encapsulated by an infection with pus formation. It is also possible that benign (thymomas) or malignant tumors (thymic carcinomas) develop.

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