Cytokines are a group of proteins involved in the innate and adaptive immune systems. Many types of cells in the body — not only immune system cells — can produce cytokines, and production is generally stimulated by the presence of an antigen. Cytokines carry signals from one cell to another, 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 a toxin. In some cases, inappropriate cytokine production takes place 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 influence several different types of cell, and can perform more than one function, while several different cytokines can perform the same function. Different types of cells can respond differently to the same cytokine and cytokines can interact with one another in various ways — for example, one might inhibit the effects of another; two together might produce a synergistic effect; and one cytokine may stimulate the production of others. Cytokines can influence the cell that produced them — these are known as autocrine — or they can influence nearby cells — these are known as paracrine. Less commonly, they might influence cells some distance away via the bloodstream — these are known as endocrine.
A number of different cytokines are involved with the innate immune system. Chemokines influence 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), known collectively as endogenous pyrogens, cause fever and an inflammatory response to infection by influencing temperature control in the hypothalamus and promoting breakdown of fat and protein to generate heat; they also stimulate production of chemokines. Interferons have a variety of functions, including preventing viral replication and activating 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 part 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) is mainly inhibitory in function, affecting multiplication of various immune cells, and also reduces fever by blocking the action of endogenous pyrogens. Interferon gamma (IFN-γ) is active in both the innate and adaptive immune responses.
In the bone marrow, another group of cytokines plays an important role. This group stimulates hematopoesis — the growth and maturing of bone marrow leukocytes into different types. Examples are interleukins 3 and 7 (IL-3 and IL-7), and colony stimulating factors (CSF).
Although 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 themselves. Excessive production of cytokines, perhaps in response to a new and unfamiliar pathogen, can result in what is known as a cytokine storm, which can cause severe and life threatening inflammation of tissue. It has been suggested that the high mortality 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 which help stimulate their growth.