Cytokine

Structure and major families

Most cytokines are made of proteins, peptides or glycoproteins, and are encoded by genes that allow rapid, regulated expression in response to stimuli. They form a large family of polypeptide regulators produced by immune cells, endothelial cells, fibroblasts and others. Common groups include:

• Interferons — antiviral and immunomodulatory mediators, often produced in response to viral infection (interferon).
• Interleukins — a diverse set originally defined as signals between leukocytes, with roles in activation and differentiation.
• Tumor necrosis factors (TNFs) — regulators of inflammation and cell death.
• Chemokines — direct the migration of immune cells to sites of infection or injury.
• Growth factors — support tissue repair and cell proliferation.

Mechanism of action

Cytokines act by binding specific cell-surface receptors, triggering intracellular signal transduction pathways. Common downstream programs include activation of kinase cascades and transcription factors such as the JAK–STAT pathway, which alter gene expression and cell behavior. Intracellular cascades amplify and integrate signals so that a single cytokine can produce effects ranging from modest changes in receptor expression to sweeping shifts in cell fate and function.

Modes of signaling and measurable effects

Signaling modes include autocrine (a cell responds to its own cytokines), paracrine (nearby cells respond), and endocrine (circulating cytokines affect distant sites). The net effect depends on concentration, receptor availability and the local cellular milieu. Cytokines can induce other cytokines, creating feedback loops that amplify or limit responses.

Detection and clinical relevance

Laboratory detection of cytokines commonly uses immunoassays such as ELISA and functional assays like ELISPOT. Clinically, cytokines are both therapeutic agents (for example interferons and certain interleukins used in cancer and viral disease) and targets of therapy (monoclonal antibodies that neutralize TNF in autoimmune disorders). Dysregulated cytokine activity contributes to conditions such as chronic inflammatory diseases and the acute "cytokine storm" seen in severe infections.

Notable distinctions and ongoing research

Although cytokines and classical hormones overlap in function, cytokines are typically produced by many cell types, act in a context-dependent manner, and form complex networks rather than linear signaling circuits. Research continues to map cytokine interactions, clarify receptor specificity, and develop interventions that modulate cytokine networks with precision for infectious disease, autoimmunity and cancer.

For further reading on specific cytokines and assays, see resources linked here: immune overview, clinical roles, biochemical nature, peptide class, glycoproteins, polypeptide regulators, interferon details, ELISA methods, ELISPOT applications, signaling pathways and signal cascades.