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Everything About Cytokines: Their Function, Structure, and Properties

Cytokines: Function, Structure, and Properties
What are cytokines? Well, they are a group of molecules that act as messengers in inter-cellular communication. They are released mainly to stimulate an immune response when the cells detect any foreign substances in the blood.
Nicks J
Last Updated: Dec 21, 2017
Cytokine Storm!
It is a life-threatening immune response in which the cytokines signal to make abnormally large number of antibodies to combat an infection. Medical conditions like acute pancreatitis are typically marked by cytokine storm.
Cytokines refer to a large group of cell-signaling molecules that essentially consists of proteins, peptides, and glycoproteins. They either have short or long chain structures. They are primarily released by immune system cells in order to initiate cell-to-cell interaction in order to generate an immune response. For instance, when white blood cells come in contact with pathogens they immediately release cytokines, which in turn communicate and activate other white blood cells to attack the foreign invader.

Thus, the immune response of alerting and recruiting more number of white blood cells (WBCs) to combat the infection is done by these chemical messengers (cytokines). In fact, cytokines are crucial in regulating the function of the immune system. Cytokines work in a similar manner in response to an injury or wound. In such circumstances, cytokines signal to increase the population of white blood cells at the injured site to speed up healing.

Cytokines are also mediators of inflammatory response that occurs after a living tissue is damaged. Inflammation caused by cytokines is a defense mechanism, which helps to keep the injury localized.
How Cytokines Initiate Cell-to-Cell Signaling?
Cell to Cell Communication
When cytokines are released from a cell, they attach to the receptors located on the outer cell membrane of target cells. This binding action triggers a chemical signal that is sent to the receptor cell, inducing it to perform a given activity. For instance, when a cell comes in contact with bacteria, it signals other cells of an imminent infection so that the immune system can work to combat the infection.
The effect of cytokines is many times visible on the cells that produce them. This is known as autocrine action. When nearby cells are affected by cytokine release, it is referred to as paracrine action. However, when secretion of cytokines has an impact on far away cells, it is referred to as endocrine action.
Types of Cytokines
Depending on their function, cytokines are classified into the following types:

Chemokines are primarily involved in stimulating white blood cells, particularly leukocytes, in response to the detection of bacteria, fungi, and viruses. Their main job is to send a signal to draw the white blood cells at the infected area. Chemokines alert the immune system about the site of infection, which is targeted by the infection-fighting cells to kill the pathogens.

Colony-stimulating Factors (CSF)
CSFs assist the body to increase the WBC count. They signal the white blood cells to multiply in order to defend the body of infections. CSFs are of two types: granulocyte colony-stimulating factors (G-CSFs) that signal bone marrow to make more number of neutrophils, the most prominent type of WBCs, while granulocyte-macrophage colony-stimulating factors (GM-CSF) stimulate the bone marrow to increase production of two types of WBCs that include neutrophils and macrophages.

Interferons do the job of activating natural killer (NK) cells, primarily to clear viral infections. NK cells are also expert at destroying tumor cells and considered as the primary mechanism to defend the immune system against cancer.

Interleukins play a key role in generating inflammatory response. They signal proliferation of T lymphocytes, which helps to contain the infection. Over 36 different interleukins have been identified that help regulate the immune response. Interleukins are named numerically starting from interleukin-1 (IL-1) to interleukin-36 (IL-36).

Transforming Growth Factors (TGFs)
They are a group of proteins that are primarily involved in generation of healthy cells. TGF-alpha, a type of transforming growth factor is secreted by macrophages brain cells. It helps in generation of epithelial tissue that lines the internal cavities of the body. TGF-beta, apart from cellular growth, are also involved in cellular differentiation, a process by which one cell type changes to another.

Tumor Necrosis Factor (TNF)
TNF are a group of proinflammatory cytokines secreted by macrophages. These cell signaling proteins play a key role in systemic inflammation. They also induce fever in response to an infection and are also involved in combating carcinogenesis and viral proliferation.

Depending on the type of cell involved in the production of cytokines, they are classified into two major types; monokines and lymphokines. The former are secreted only by monocytes, while the later are released by lymphocytes. Both monocytes and lymphocytes are a type of WBCs, that help in regulating immunity.
Cytokines can also be classified as pro-inflammatory and anti-inflammatory. While the pro-inflammatory ones induce inflammation in response to tissue injury, the function of anti-inflammatory cytokines {(IL-4), (IL-10), and (IL-13)} is exactly the opposite. Their purpose is to lower the inflammatory response. Chronic inflammation, such as in rheumatoid arthritis, is harmful to the health, hence to mitigate this inflammatory action, anti-inflammatory cytokines are released into the body.