Cytokines are proteins that regulate the immune system’s response to threats. They can affect different cell types and perform multiple functions. Some cytokines are involved in the innate immune system, while others play a role in the adaptive immune system. Excessive cytokine production can cause severe tissue inflammation and lead to life-threatening conditions.
Cytokines are a group of proteins involved in both the innate and adaptive immune systems. Many types of cells in the body, not just cells of the immune system, can produce cytokines and production is usually stimulated by the presence of an antigen. Cytokines carry signals from cell to cell, alter cell behavior in various ways, and regulate the body’s immune response to a potential threat, which could be a pathogen, such as a virus, bacterium, or parasite or toxin. In some cases, the inappropriate production of cytokines occurs in response to something harmless, resulting in an allergic reaction. Usually, cytokine proteins are not stored ready-made, but are synthesized when needed.
The role of these proteins in the immune response is complex. A single cytokine can affect different cell types and can perform more than one function, while several different cytokines can perform the same function. Different cell types can respond differently to the same cytokine, and cytokines can interact with each other in various ways: for example, one might inhibit the effects of another; two together could produce a synergistic effect; and one cytokine can stimulate the production of others. Cytokines can affect the cell that made them – these are known as autocrines – or they can affect neighboring cells – these are known as paracrines. Less commonly, they might affect cells some distance away through the bloodstream – these are known as endocrine cells.
Several cytokines are involved in the innate immune system. Chemokines affect the movement of immune cells by chemotaxis and can attract these cells to sites of injury or infection. Tumor necrosis factor-alpha (TNF-α), interleukin 1 (IL-1), and interleukin 6 (IL-6), collectively known as endogenous pyrogens, cause fever and an inflammatory response to infection by affecting control of the temperature in the hypothalamus and aiding the breakdown of fats and proteins to generate heat; they also stimulate the production of chemokines. Interferons have a variety of functions, including the prevention of viral replication and the activation of macrophages and NK cells. Interleukin 10 (IL-10), in contrast, has an essentially inhibitory effect on the immune response.
Other types of cytokines play a role in the adaptive immune system. Interleukins 2, 4 and 5 (IL-2, IL-4 and IL-5) stimulate the growth and development of a variety of immune cells. Transforming growth factor beta (TGF-β) has a mainly inhibitory function, affecting the multiplication of various immune cells, and also reduces fever by blocking the action of endogenous pyrogens. Interferon-gamma (IFN-γ) is active in both innate and adaptive immune responses.
In the bone marrow, another group of cytokines play an important role. This group stimulates hematopoesis, growth and maturation of bone marrow leukocytes in different types. Examples are interleukins 3 and 7 (IL-3 and IL-7) and colony stimulating factors (CSF).
While these proteins play a vital role in coordinating the immune response to eliminate threats, they can sometimes aggravate conditions caused by pathogens or cause disease itself. Excessive cytokine production, perhaps in response to a new and unknown pathogen, can result in what is known as a cytokine storm, which can cause severe and life-threatening tissue inflammation. It has been suggested that the high death rate during the 1918 influenza pandemic was due to this effect rather than the direct effects of the virus itself. Other related problems include toxic shock, allergies and autoimmune diseases. Some cancer cells produce cytokines that help stimulate their growth.
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