The term cytokines encompasses a highly differentiated group of peptides and proteins that act as messengers to exert substantial influence on immune responses by cells of the innate and acquired immune systems. Cytokines include interleukins, interferons, tumor necrosis factors and other polypeptides or proteins. Cytokines are mostly-but not exclusively-produced by cells of the immune system and dock onto specific receptors on the various cells of the immune system to achieve the necessary activation of target cells.
What are cytokines?
The human immune system consists mainly of two components, the nonspecific, genetically fixed system and the adaptive, acquired, immune defense. The genetically fixed component of the immune system can respond within minutes. This includes, for example, inflammatory responses and phagocytosis. The adaptive immune defense is much slower in its immune responses, but its advantage is to be able to adapt to challenges from new pathogens to which the innate immune defense has no response. The cells of both parts of the immune system must respond quickly and appropriately to unpredictable situations by killing pathogenic germs or degrading harmful substances, comparable to police duties. The necessary control of the immune cells involved is taken over by cytokines, which are mostly released by the immune cells themselves. They are proteins or polypeptides that dock as messenger substances to specific receptors of the target cells. The cytokines do not need to enter the target cell to cause the cell to make the required response. For example, the “message” of a cytokine may include the stimulation to proliferate by division, to proliferate, or the instruction to differentiate into an active stage.
Anatomy and structure
Immune responses are highly differentiated and complex, so that the control of the immune system, by analogy, must also consist of differentiated messages or instructions. Because each messenger can transmit only one specific instruction to specified target cells at a time, the number of known messengers that are categorized as cytokines is very large. Five different groups of substances make up the class of cytokines. These are interferons (IFN), interleukins (IL), colony-stimulating factors (CSF), tumor necrosis factors (TNF) and chemokines. Interferons, interleukins and substances classified as colony-stimulating factors are mostly relatively short-chain proteins or polypeptides formed from about one hundred to six hundred amino acids. The group of chemokines is composed of even shorter-chain proteins with less than 100 to a maximum of 125 amino acids, so that they are almost all polypeptides. A common property of cytokines is that they do not need to enter the cell to be stimulated, but merely dock at specific receptors protruding from the cell membrane to become effective.
Function and tasks
The individual substances that belong to one of the cytokine substance groups have different functions and tasks. However, all activities can be related to the control and influence of the inherited and acquired immune system. Interferons are mainly secreted by leukocytes such as macrophages and monocytes. They stimulate cells to produce special proteins that have antiviral and antitumor properties and thus an immunostimulatory effect. Interleukins enable white blood cells (leukocytes) to communicate with each other and, together with tumor necrosis factor alpha, control concentrated defense and inflammatory reactions. This includes systemic effects such as triggering fever and increases in permeability, some of which can lead to dangerous conditions when blood is allowed to enter tissues due to increased permeability of blood vessels. Colony-stimulating factors include growth factors for white and red blood cells. Substances such as erythropoietin (EPO), which is also known as a banned doping agent, and thrombopoietin are among them. Tumor necrosis factor is the name given to a multifunctional messenger substance released mainly by macrophages. TNF can control the activities of various immune cells.TNF, for example, can induce apoptosis (cell death), but also cell proliferation, cell differentiation and the release of other cytokines. Chemokines consist of small signaling proteins that can cause cells to migrate toward the highest concentration of chemokines. Such migratory movements become visible at local sites of inflammation with an accumulation of certain immune cells.
Diseases
The highly differentiated and complex control by cytokines already lead to the expectation that there may also be erroneous responses with effects on the immune system. For example, the immune responses may be too weak or too strong, or they may be misdirected. The disturbances of the immune system can occur endogenously, i.e. without recognizable influence from the outside, or also due to the effect of pathogenic germs or toxic substances. Typical overreactions of the immune system with mild to severe health impairments are allergic reactions. A special form of an allergic immune response is anaphylactic shock, which can develop in a very short time from a local immune reaction to a systemic reaction with a life-threatening condition due to the release of large amounts of inflammation-triggering messenger substances. Just as well known as allergic overreactions of the immune system are misdirected autoimmune reactions, which are directed against the body’s own tissue because the tissue cells cannot “identify” themselves properly and are therefore regarded as foreign to the body or because cytokines cannot classify the cells as the body’s own due to their own malfunctions. Typical and relatively common autoimmune diseases are polyarthritis and rheumatoid arthritis. There is an increased accumulation of interleukin-1 in the joints, so that cartilage substance is degraded more than it is built up. Similar processes can occur in the bones when the osteoclasts that break down bone are increasingly activated without the osteoblasts that build up bone being able to compensate for the breakdown. An example of aberrant immune responses caused by pathogenic germs is the acquired immunodeficiency AIDS, which is triggered by the HIV virus via the attack of T-helper cells.
