A cytokine is a protein produced by immune or non-immune cells that influences the behavior of other cells through specific receptors. Cytokines are central messengers of the immune system, coordinating defense against infection, inflammation, and immune regulation. They can stimulate or restrain immune activity, and their effects depend on the responding cell and its physiological context. Interleukins, interferons, and colony-stimulating factors are among the groups commonly included under this term. (cancer.gov)
Production and modes of action
Cytokines are produced by many cell types, rather than by a single specialized organ. Sources include leukocytes, such as activated T cells, as well as endothelial cells and fibroblasts. Production often follows recognition of infection or another activating stimulus. Different cell populations release different combinations of cytokines, allowing communication between immune cells and the surrounding tissue. (ncbi.nlm.nih.gov)
Cytokines can act through three overlapping modes of communication. In autocrine signaling, a cytokine affects the cell that produced it; in paracrine signaling, it affects neighboring cells; and in endocrine signaling, it reaches distant targets through the circulation. Local effects are particularly important, but circulating cytokines can also coordinate body-wide responses. Their range of action depends partly on access to the bloodstream and persistence there. (ncbi.nlm.nih.gov)
The distinction between a cytokine and a hormone is therefore not an absolute difference in signaling mechanism. Classical endocrine hormones are defined by secretion into circulation and action at distant tissues, whereas cytokines are commonly discussed in relation to cellular interactions and immune responses. Some proteins occupy overlapping functional categories. (ncbi.nlm.nih.gov)
Major groups and terminology
Cytokine names reflect a mixture of discovery history, biological activity, and molecular relationships. They do not constitute a single structurally uniform family. Classification may emphasize the cytokine’s structure, its receptor family, or its principal biological functions. (ncbi.nlm.nih.gov)
- Interleukins were originally named as messengers between leukocytes. The designation persists, although many are produced by or act on non-leukocyte cells. Individual interleukins have widely differing functions. (ncbi.nlm.nih.gov)
- Interferons participate in antiviral defense and immune regulation. Type I interferons, including interferon-α and interferon-β, induce cellular programs that inhibit viral replication. (ncbi.nlm.nih.gov)
- Chemokines are a cytokine family especially associated with directing cell migration. Cells bearing appropriate receptors move toward chemokine sources, helping position immune cells within tissues. (ncbi.nlm.nih.gov)
- Tumor necrosis factor and related molecules belong to a distinct family involved in immune and inflammatory responses; some members occur in membrane-bound forms. (ncbi.nlm.nih.gov)
- Colony-stimulating factors regulate blood-cell production and differentiation, linking cytokine activity to the maintenance and replenishment of immune-cell populations. (ncbi.nlm.nih.gov)
Receptors and intracellular signaling
A cytokine functions as a ligand for a cytokine receptor, usually at the cell surface. Binding initiates intracellular signaling rather than requiring the cytokine itself to enter the cell. Whether a cell responds depends on its receptor expression and the availability of downstream signaling components. Several cytokines can share receptor subunits, while different receptor complexes can modify the effects of a particular cytokine. (pmc.ncbi.nlm.nih.gov)
Many cytokine receptors activate the JAK–STAT signaling pathway. Cytokine binding assembles or rearranges receptor complexes, bringing associated Janus kinases into an activating configuration. These kinases phosphorylate receptor-associated proteins and STAT proteins. Activated STATs enter the cell nucleus and function as transcription factors, altering gene expression. Receptor abundance, binding affinity, and signaling duration help determine the resulting response. (pubmed.ncbi.nlm.nih.gov)
Not every cytokine receptor uses this pathway. Chemokine receptors generally signal through coupled G proteins. Thus, “cytokine signaling” encompasses several molecular systems rather than one universal mechanism. (ncbi.nlm.nih.gov)
Biological functions
Cytokines connect innate immunity with adaptive immunity. During infection, cytokines released after pathogen recognition activate local defenses and recruit additional immune cells. Type I interferons induce antiviral responses in infected cells and their neighbors. Chemokines help bring responsive leukocytes to sites where their effector functions are needed. (ncbi.nlm.nih.gov)
Other cytokines shape lymphocyte responses. Interleukin-2, produced mainly by activated T cells, supports T-cell expansion and also acts on B cells and other immune populations. Interleukin-12 promotes interferon-γ production and helps direct particular T-cell responses. These activities show how cytokines influence both the size and character of an immune response. (ncbi.nlm.nih.gov)
Cytokines also restrain inflammation. Interleukin-10 and transforming growth factor beta have important regulatory roles. Such regulation contributes to immune homeostasis, although neither molecule has an exclusively suppressive effect in every setting. (pubmed.ncbi.nlm.nih.gov)
Pleiotropy and redundancy
Two characteristic properties are pleiotropy, in which one cytokine produces several different effects, and redundancy, in which different cytokines produce overlapping effects. Consequently, a cytokine cannot be assigned a single biological function independently of context. Interleukin-10, for example, can suppress inflammatory cytokine production by some cells while enhancing particular T-cell effector functions. (pmc.ncbi.nlm.nih.gov)
These properties arise partly from shared receptors and signaling machinery. Cellular state, receptor trafficking, and the abundance of intracellular components further influence the outcome. Overlapping activity does not imply complete interchangeability: different cytokines can retain distinct functions despite activating common pathways. (pmc.ncbi.nlm.nih.gov)
Medical significance
Laboratory-produced cytokines are used in some treatments for cancer and other diseases. Cytokine pathways can also be targeted to modify immune activity. Their broad and context-dependent effects are important considerations in cytokine-based drug development. (cancer.gov)
Excessive cytokine release can produce systemic illness. Cytokine release syndrome may occur after certain immunotherapies, including some monoclonal antibody treatments and CAR-T-cell therapy. Manifestations include fever, low blood pressure, and breathing difficulties; severity ranges from mild reactions to life-threatening disease. (cancer.gov)