Cytokines are small proteins secreted by immune cells and many non‑immune cells that act as intercellular messengers. They bind to specific receptors on target cells to regulate immunity, inflammation, and blood cell formation.
Explanation
Cytokines form a complex network of signalling molecules that coordinate the innate and adaptive immune responses. Produced by leukocytes, stromal cells, endothelial cells, and many tissues in response to stimuli, they exert their effects in autocrine, paracrine or, less commonly, endocrine fashions. Cytokines include interleukins, interferons, tumour necrosis factors, chemokines, colony‑stimulating factors and growth factors. When a pathogen is detected, innate immune cells such as macrophages and dendritic cells release pro‑inflammatory cytokines like interleukin‑1, interleukin‑6 and tumour necrosis factor‑α that promote fever, recruit leukocytes and enhance antigen presentation. Interferons, particularly type I interferons, induce antiviral states in neighbouring cells by up‑regulating genes that limit viral replication. In the adaptive response, cytokines produced by T helper subsets drive differentiation and function of other lymphocytes; for example, interleukin‑2 is critical for T‑cell proliferation, whereas interleukin‑4 promotes antibody class switching in B cells. Cytokines also regulate haematopoiesis through colony‑stimulating factors such as G‑CSF and GM‑CSF. Dysregulated cytokine production contributes to autoimmune and inflammatory disorders and to severe systemic responses known as cytokine storms, seen in sepsis and some viral infections. Therapeutics that block specific cytokines, like anti‑TNF antibodies, have become important treatments for diseases such as rheumatoid arthritis.
Notable cytokines and functions
Interleukin‑2 is a growth factor for activated T cells and is used therapeutically to stimulate immune responses in cancer. Interferon‑γ, produced by natural killer cells and T helper 1 cells, activates macrophages and enhances antigen presentation. Interleukin‑10 serves as an anti‑inflammatory mediator by suppressing cytokine production and limiting tissue damage. Chemokines such as CXCL8 (interleukin‑8) guide neutrophils to infection sites, whereas CCL2 recruits monocytes. Tumour necrosis factor‑α and interleukin‑1 are central mediators of fever and acute phase responses but can cause tissue injury when produced in excess. Therapeutic recombinant cytokines, including erythropoietin and G‑CSF, stimulate blood cell production in patients with anaemia or neutropenia.
Cytokines are fundamental regulators of immune and inflammatory responses. Their diverse actions enable precise communication between cells, but imbalances can lead to pathology, making cytokines both important biomarkers and therapeutic targets.
Related Terms: Interleukin, Chemokine, Interferon, Tumour necrosis factor, Cytokine storm