Most people know the thymus only as sweetbreads from the menu. But it plays a very important role for 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 exactly is it located?
The thymus is also called the thymus gland or sweetbread. It is located in our rib cage just behind the sternum above the pericardium and extends approximately from the base of the clavicles to the fourth pair of ribs. Weighing only about 40 g, it is a lightweight among 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 surrounded by a connective tissue capsule. From this septa (a kind of partition) extend into the interior and divide off individual lobules (lobuli thymi). The lobules show a lighter medullary zone (medulla), which is surrounded by a darker cortex. In the medulla one finds the Hasall corpuscles, which are characteristic for the thymus. Mainly in the cortex are stored so-called thymic lymphocytes (also thymocytes), which are so important for our immune defense.
What are the functions of the thymus?
In classical antiquity, the thymus was still considered the seat of the soul. Its name is derived from the Greek word thymos (life energy). Meanwhile, we know that its main function is the development of the immune system. That is why the thymus gland is called a primary lymphatic organ, just like the bone marrow.
The stem cells – which are cells whose function has been established but which have yet to develop – migrate from the bone marrow via the bloodstream into the thymus, where they mature into T lymphocytes or T cells (T = thymus) – this process is called imprinting. The stem cells pass through the thymic lobules from the outside to the inside.
In the process, they “learn” to distinguish between the body’s own and foreign antigens, i.e. structures on the surface of cells. This is important so that the T lymphocytes can later recognize and destroy bacteria, viruses, parasites or even tumor cells, but spare the body’s own cells. The thymus is thus a kind of school for the defense cells, in which they are trained to become ready “body policemen”.
After imprinting, the T cells migrate from the thymus to the lymph nodes, where they await deployment. Each T lymphocyte is specialized for a specific antigen. As soon as it recognizes this antigen in an intruder, this T lymphocyte multiplies, it is “cloned”, so to speak. Then the cells foreign to the body are destroyed and thus, for example, an infection is fought off. The thymus is rightly also 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 throughout life
In the newborn, each lobe of the thymus is about 5 cm long and 2 cm wide. The organ continues to grow somewhat until puberty, when it weighs about 40 g.
As life progresses, the thymus then shrinks and most of the lymphoid tissue is replaced by adipose tissue – this process is called involution. Medullary and cortical tissues decrease and the number of Hasall bodies also decreases. The tasks of the thymus are then taken over by secondary lymphoid organs such as lymph nodes or spleen.
As late as the beginning of the 20th century, the involution of the thymus was held responsible for the aging processes in humans – a hypothesis that could not be confirmed in this way.
