Cytokines are a broad and loose category of small proteins (~5–25 kDa) important in cell signaling. Cytokines are peptides and cannot cross the lipid bilayer of cells to enter the cytoplasm. Cytokines have been shown to be involved in autocrine, paracrine and endocrine signaling as immunomodulating agents. Their definite distinction from hormones is still part of ongoing research. The word comes from the ancient Greek language: cyto, from Greek κύτος, kytos, ‘cavity, cell’ + kines, from Greek κίνησις, kinēsis, ‘movement’.