**Cytokines** are a broad and loose category of small [proteins](https://en.wikipedia.org/wiki/Protein "Protein") (~5–25 [kDa](https://en.wikipedia.org/wiki/KDa "KDa") important in [cell signaling](https://en.wikipedia.org/wiki/Cell_signaling "Cell signaling"). Cytokines are [peptides](https://en.wikipedia.org/wiki/Peptide "Peptide") and cannot cross the [lipid bilayer](https://en.wikipedia.org/wiki/Lipid_bilayer "Lipid bilayer") of cells to enter the [cytoplasm](https://en.wikipedia.org/wiki/Cytoplasm "Cytoplasm"). Cytokines have been shown to be involved in [autocrine](https://en.wikipedia.org/wiki/Autocrine_signaling "Autocrine signaling"), [paracrine](https://en.wikipedia.org/wiki/Paracrine_signaling "Paracrine signaling") and [endocrine signaling](https://en.wikipedia.org/wiki/Endocrine_signaling "Endocrine signaling") as [immunomodulating agents](https://en.wikipedia.org/wiki/Immunomodulation "Immunomodulation"). Their definite distinction from [hormones](https://en.wikipedia.org/wiki/Hormones "Hormones") is still part of ongoing research. Cytokines include [chemokines](https://en.wikipedia.org/wiki/Chemokine "Chemokine"), [interferons](https://en.wikipedia.org/wiki/Interferon "Interferon"), [interleukins](https://en.wikipedia.org/wiki/Interleukin "Interleukin"), [lymphokines](https://en.wikipedia.org/wiki/Lymphokine "Lymphokine"), and [tumour necrosis factors](https://en.wikipedia.org/wiki/Tumour_necrosis_factor "Tumour necrosis factor"), but generally not hormones or [growth factors](https://en.wikipedia.org/wiki/Growth_factor "Growth factor") (despite some [overlap in the terminology](https://en.wikipedia.org/wiki/Growth_factor#cytokine "Growth factor")). Cytokines are produced by a broad range of cells, including immune cells like [macrophages](https://en.wikipedia.org/wiki/Macrophage "Macrophage"), [B lymphocytes](https://en.wikipedia.org/wiki/B_cell "B cell"), [T lymphocytes](https://en.wikipedia.org/wiki/T_cell "T cell") and [mast cells](https://en.wikipedia.org/wiki/Mast_cell "Mast cell"), as well as [endothelial cells](https://en.wikipedia.org/wiki/Endothelium "Endothelium"), [fibroblasts](https://en.wikipedia.org/wiki/Fibroblast "Fibroblast"), and various [stromal cells](https://en.wikipedia.org/wiki/Stromal_cell "Stromal cell"); a given cytokine may be produced by more than one type of cell. They act through [cell surface receptors](https://en.wikipedia.org/wiki/Cell_surface_receptor "Cell surface receptor") and are especially important in the [immune system](https://en.wikipedia.org/wiki/Immune_system "Immune system"); cytokines modulate the balance between [humoral](https://en.wikipedia.org/wiki/Humoral_immunity "Humoral immunity") and [cell-based](https://en.wikipedia.org/wiki/Cell-mediated_immunity "Cell-mediated immunity") immune responses, and they regulate the maturation, growth, and responsiveness of particular cell populations. Some cytokines enhance or inhibit the action of other cytokines in complex ways. They are different from [hormones](https://en.wikipedia.org/wiki/Hormone "Hormone"), which are also important cell signaling molecules. Hormones circulate in higher concentrations, and tend to be made by specific kinds of cells. Cytokines are important in health and disease, specifically in host [immune responses](https://en.wikipedia.org/wiki/Immune_response "Immune response") to [infection](https://en.wikipedia.org/wiki/Infection "Infection"), [inflammation](https://en.wikipedia.org/wiki/Inflammation "Inflammation"), [trauma](https://en.wikipedia.org/wiki/Major_trauma "Major trauma"), [sepsis](https://en.wikipedia.org/wiki/Sepsis "Sepsis"), [cancer](https://en.wikipedia.org/wiki/Cancer "Cancer"), and reproduction.