The thymus

Most people know the thymus only as sweetbreads from the menu. But it plays a very important role in our immune system: In the thymus, our white blood cells "learn" to recognize and destroy foreign cells.

What does the thymus look like and where is it exactly?

The thymus is also called thymus or Bries. It lies in our thorax directly behind the breastbone above the pericardium and extends from the approach of the clavicles to the fourth pair of ribs. With only about 40 g it is a lightweight among the organs.

The thymus was first described in the 16th century by Berengario de Carpi, a great anatomist of the time, who taught in Rome, Padua and Bologna.

The thymus gland consists of a left and a right lobe, which are surrounded by a connective tissue capsule. From this draw septa (a kind of partitions) into the interior and divide off individual lobules (Lobuli thymi). The lobules show a lighter medulla surrounded by a darker cortex. In the medulla one finds the Hasall bodies, which are characteristic for the Thymus. Mainly in the bark so-called thymus lymphocytes (also thymocytes) are embedded, which are so important for our immune system.

What functions does the thymus have?

In classical antiquity, the thymus was still regarded as the seat of the soul. Its name is derived from the Greek word thymos (life energy). We now know that his main task is the development of the immune system. Therefore, the thymus gland, just like the bone marrow, is called the primary lymphatic organ.

The stem cells - these are cells whose function is established, but which still have to develop - migrate from the bone marrow via the bloodstream into the thymus and mature there to T lymphocytes or T cells (T = thymus) - this process is called imprinting. The stem cells pass through the thymus lobules from outside to inside.

In doing so, they "learn" to distinguish the body's own from foreign antigens, ie structures on the surface of cells. This is important so that the T-lymphocytes later recognize and destroy bacteria, viruses, parasites or even tumor cells, but spare the body's own cells. The thymus is a kind of school for the defense cells, in which they are trained to become "body policemen".

After imprinting, the T cells migrate from the thymus to the lymph nodes and wait for their use. Every T lymphocyte specializes in a specific antigen. As soon as he recognizes this in an intruder, this T-lymphocyte multiplies, it is "cloned", so to speak. Then the alien cells are destroyed and so, for example, warded off an infection. The thymus is also rightly called the thymus gland: it produces the hormones thymosin, thymopoietin I and II, which are important for the maturation of T lymphocytes.

The thymus changes in the course of life

In the newborn, each lobe of the thymus is about 5 cm long and 2 cm wide. Until puberty, the organ grows a bit until it weighs about 40 g.

In the course of further life, the thymus then shrinks and the lymphatic tissue is largely replaced by adipose tissue - this process is called involution. Marrow and bark tissue decreases and the number of Hasall bodies is also reduced. The tasks of the thymus then take over secondary lymphatic organs such as lymph nodes or spleen.

As early as the beginning of the twentieth century, the thymus involution was blamed for human aging, a hypothesis that could not be confirmed.

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