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 here so that they can distinguish the body’s own surface structures (antigens) of, for example, bacteria or viruses from foreign antigens. This is important to prevent the immune cells from attacking their own body and causing so-called autoimmune diseases.
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 pass through the lobes and divide the thymus into many small lobules called lobuli thymi. Each lobule consists of a pale medullary zone (medulla) surrounded by a darker cortex.
The medullary zone of the thymus contains the characteristic Hassall bodies. They are easily recognized, especially visually under the microscope. The Hassall corpuscles probably consist of cover tissue cells (epithelial cells) arranged together and look like small onions due to this layering. Their function is not yet clear, but it is suspected that they assist in the maturation of immune cells.
Change of the thymus gland
In the newborn, the thymus cicra is five centimeters long and two centimeters wide. In the course of childhood up to puberty, the thymus reaches its maximum weight of 35 to 50 grams. From sexual maturity, the thymus shrinks. Function and tissue change. In old age, predominantly fat and connective tissue is found there, and the weight is reduced to about three grams. This process is called thymic involution. However, most of the formation of the immune cells is already completed before that.
After its regression, the secondary lymphoid organs (lymph nodes, spleen) take over the functions of the thymus.
What is the function of the thymus?
The thymus, together with the bone marrow, is called the primary lymphoid organ. This means that the immune system develops and matures in the thymus and bone marrow.
For this purpose, the immune cells pass through several stations:
Bone marrow
Multipotent stem cells” migrate from the bone marrow; these 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 obtain imprinting and differentiation, the progenitor cells (thymocytes) must pass through the thymus from the cortex to the medullary region and then be released back into the bloodstream as T lymphocytes.
The imprinting process takes place in three steps. Subsequently, those cells are sorted out that have not been “trained” correctly or not well enough. In this process, more than 90 percent of the imprinted cells are eliminated.
At the end of the imprinting and selection process, the remaining T lymphocytes have learned to distinguish endogenous from exogenous tissue by recognizing the surface structures accordingly. They can later identify and attack bacteria, viruses, parasites or tumor cells, while the body’s own cells are spared.
Transfer to the lymph nodes
After their “training”, the T-lymphocytes are released back into the blood and thus reach the lymph nodes. There they wait to be used. If a T cell recognizes its very special surface molecule in an intruder, this T cell multiplies. Together, the clones attack bacteria, for example. This is how an infection is fought off.
Thymus gland: hormone production
Why is this organ also called the thymus gland? 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 T lymphocytes in the thymus.
Where is the thymus located?
What problems can the thymus cause?
Due to the complex structure of the thymus, abnormalities may occur more frequently. However, this does not necessarily mean that its function is impaired. If, then impairment plays a role especially at a young age when the thymus is active.
For example, there are congenital disorders in which the thymus is not developed at all (thymic aplasia) or is only partially developed. This developmental disorder can lead to pronounced immunodeficiencies with high susceptibility to infection. Thymic aplasia often accompanies other hereditary defects, such as DiGeorge syndrome, retinoid embryopathy, Louis-Bar syndrome or Wiskott-Aldrich syndrome.
Especially in early infancy, the thymus may enlarge (persistent thymic hyperplasia) and press on the trachea, causing breathing difficulties. In most cases, however, this 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.