T lymphocytes are a component of white blood cells that are responsible for immune defense. Their main function is to detect and fight pathological changes in the cell turn in the form of viruses or bacteria.
What is a T lymphocyte?
T lymphocytes, or also known as T cells, is the name given to a component of white blood cells that controls the immune response. The abbreviation “T” stands for thymus. The thymus represents an organ of the lymphatic system in which, among other things, the T lymphocytes mature. Together with the B lymphocytes, the T lymphocytes constitute the specific or adaptive immune response. All cells of the blood are produced within the bone marrow, thus also the T lymphocytes. From the spinal cord, the T lymphocytes migrate to the thymus, where the receptors of the main tissue compatibility complex are formed. Subsequently, T lymphocytes are segregated to fight not only exogenous antigens but also endogenous proteins. However, T cells can only recognize and fight exogenous antibodies if they have already bound to the MHC (major tissue compatibility complex). Unbound antibodies can only be recognized by T cells if they are actively displayed by antigen-presenting cells (MHC restriction).
Anatomy and structure
T lymphocytes have a spherical shape and are about the same size as erythrocytes (red blood cells). The size grasps a diameter of about 7.5 µm. The red and white blood cells cannot be distinguished under a microscope. Only immunohistology or antibody staining can reveal the T cells. The chromosome assembly within the round and slightly indented nucleus is stainable and appears dense and strong. The plasma thumb, consisting of cytoplasm, wraps around the nucleus of the cell and is barely perceptible under a light microscope. The cell organelles can be seen in the form of azurophil granules. The cell substance of the T cell consists of many free ribosomes. Ribosomes are referred to as macromolecular complexes consisting of proteins and ribonucleic acid. As subtypes of T lymphocytes, 6 other cell types are categorized:
1. helper T cells
2. cytotoxic T cell
3. regulatory T cells
4. memory T cells
5. natural killer T cells – NK T cells
6. γδ-antigen receptor-positive T lymphocytes.
Function and tasks
T lymphocytes distribute throughout the organism via the blood and guard the membrane composition of the body’s cells for pathological changes. If bacteria or viruses enter the organism, they bind to the cell surfaces and thus change their substance. The MHC molecules check the individual passing receptors for their forms and tasks and are activated in case of a match. The activation is caused by the antigen receptors and coreceptors. Depending on the type of pathological changes, specific T lymphocytes are activated in their function. Thus, the different mechanisms can be activated by the T-killer cells (destroy pathological cells directly), T-helper cells (attract further immune cells by the release of soluble messenger substances) or regulatory T-cells (prevent excessive reactions to endogenous and healthy cells). The main task of the T lymphocytes is thus the targeted damage of pathological changes through the formation of chemical substances as an immune reaction. The reactions vary in intensity. This depends on the stimulating antigen and the form of the pathological change. The non-activated T lymphocytes move in the area of blood and lymphatic tissue. They move creeping in this area, but have membrane proteins and receptors for small signaling proteins. The T lymphocytes leave the blood stream via endothelial niches of the postcapillary venules and thus enter the tissue structures. With the lymphatic fluid, they empty into the left venous angle via the thoracic duct. Alternatively, T lymphocytes may migrate to a lymphoid organ via endothelial niches of a high-endothelial venule. The specific characteristic function of T lymphocytes is expressed in the release of substances to affect metabolism within bone.
Diseases
In the case of a disorder of the immune system, a distinction is made between congenital immunodeficiencies and acquired immunodeficiencies.In congenital immunodeficiencies, the T lymphocytes and the B lymphocytes are affected. The cellular and humoral immune defense is damaged, this is called a severe combined immunodeficiency. In the long term, such a disorder can only be treated with a bone marrow transplant in order to give these patients a chance of survival. Furthermore, congenital immunodeficiencies include Di-George syndrome and nude lymphocyte syndrome. Acquired immunodeficiency is acquired in the course of life. This can be due to disease, malnutrition or harmful environmental influences. Drug therapies can also cause an acquired defect. Infection such as HIV (human immunodeficiency virus), HTLV I virus (human T-cell leukemia virus 1), and HTLV II virus (human T-cell leukemia virus type 2) cause immune deficiency and can cause AIDS, adult T-cell leukemia, and tropical spastic paraparesis. In addition, hypersensitivity reactions may occur in the form of an immune overreaction. This is called an allergic reaction and is triggered by harmless antigens such as dust, pollen, food or drugs. Chronic autoimmune diseases are also common. Here, the immune defense is directed against the body’s own cells and structures. Common autoimmune diseases include diabetes mellitus type I, rheumatoid arthritis and multiple sclerosis (MS). However, certain drugs also influence the function of T lymphocytes. These include immunosuppressants and cytostatics, for example. Radiation therapies to combat tumors also kill the white blood cells. In tumor diseases in the form of malignant lymphomas and acute lymphatic leukemia (often in children), the T lymphocytes degenerate. Therapeutic options are often limited in these cases.
