Cytokines constitute a heterogeneous class of low-molecular-weight signaling proteins that mediate both the induction and effector phases of innate and adaptive immune responses. They regulate cell proliferation, differentiation, intercellular communication, and apoptosis, and are synthesized by a broad range of hematopoietic and non-hematopoietic cell lineages. Cytokines typically signal in an autocrine or paracrine manner, binding cell-surface receptors at picomolar to nanomolar concentrations to initiate downstream signal transduction cascades (e.g., JAK-STAT, MAPK, NF-κB pathways).
From a functional classification standpoint, cytokines are more accurately grouped with hormones than with growth factors, given their capacity to exert pleiotropic and often systemic effects at trace concentrations. This distinction is not absolute, however: several cytokines — including IL-2, IL-6, and TGF-β family members — exhibit substantial growth factor–like activity, underscoring the mechanistic overlap between these signaling classes.
Growth factors, in contrast, are polypeptides that engage specific cell-surface receptors — frequently receptor tyrosine kinases — to drive mitogenic signaling and/or lineage-specific differentiation. Their downstream effects are transduced through pathways such as RAS-RAF-MEK-ERK and PI3K-AKT. Growth factor activity spans a spectrum of specificity: some ligands (e.g., EGF, PDGF) act as broad mitogens across multiple cell types, while others (e.g., erythropoietin, G-CSF) exhibit lineage-restricted activity confined to a single hematopoietic compartment.