Cytokine

Cytokines are a broad category of small secreted proteins or glycoproteins that mediate and regulate immunity, inflammation, and hematopoiesis. They are produced by a variety of cell types, especially immune cells such as macrophages, T lymphocytes, and dendritic cells, but also by endothelial cells, fibroblasts, and other tissues. Cytokines act as signaling molecules that influence the behavior, differentiation, and survival of target cells bearing specific receptors. They can act locally (autocrine or paracrine actions) or remotely (endocrine actions).

Cytokines are classified into several families, including interleukins, interferons, chemokines, transforming growth factor beta, tumor necrosis factors, and colony-stimulating factors. Examples: IL-2 promotes T cell proliferation; IL-6 drives fever and acute-phase responses; IFN-γ activates macrophages; TNF-α promotes inflammation; IL-8 (CXCL8) attracts neutrophils.

Binding of cytokines to their receptors activates intracellular signaling pathways such as JAK-STAT and MAP kinases, leading to changes in gene expression that regulate immune responses, cell growth, and tissue repair.

Physiological and clinical relevance: cytokines coordinate host defense but excessive or dysregulated production can cause inflammatory or autoimmune diseases (such as rheumatoid arthritis, inflammatory bowel disease) and can contribute to septic shock. Cytokine therapies (e.g., interferons, interleukin-2) and cytokine antagonists (e.g., anti-TNF antibodies) are used to treat infections, cancer, and autoimmune disorders. A notable risk in some settings is the cytokine storm, a hyperactive systemic inflammatory reaction.

History: the term cytokine emerged in the 1970s to describe secreted factors involved in immune signaling, with many cytokines identified in subsequent decades.