Cytokines are a broad and loose category of small proteins (~5–25 kDa)
important in cell signaling. Due to their size, cytokines cannot cross the
lipid bilayer of cells to enter the cytoplasm and therefore typically exert
their functions by interacting with specific cytokine receptors on the target
cell surface. Cytokines have been shown to be involved in autocrine, paracrine
and endocrine signaling as immunomodulating agents.
Cytokines include chemokines, interferons, interleukins, lymphokines, and
tumour necrosis factors, but generally not hormones or growth factors (despite
some overlap in the terminology).