They act through receptors, and are especially important in the immune system; cytokines modulate the balance between humoral and cell-based 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, which are also important cell signaling molecules, in that hormones circulate in much lower concentrations and hormones tend to be made by specific kinds of cells.
They are important in health and disease, specifically in host responses to infection, immune responses, inflammation, trauma, sepsis, cancer, and reproduction.
In 1969 Dudley Dumonde proposed the term "lymphokine" to describe proteins secreted from lymphocytes and later, proteins derived from macrophages and monocytes in culture were called "monokines". As scientists learned more, it was understood that these proteins and others were part of a broader class of proteins involved in self-defense, and should be called "cytokines".
Difference from hormones
Classic hormones circulate in nanomolar (10-9 M) concentrations that usually vary by less than one order of magnitude. In contrast, some cytokines (such as IL-6) circulate in picomolar (10-12 M) concentrations that can increase up to 1,000-fold during trauma or infection. The widespread distribution of cellular sources for cytokines may be a feature that differentiates them from hormones. Virtually all nucleated cells, but especially endo/epithelial cells and resident macrophages (many near the interface with the external environment) are potent producers of IL-1, IL-6, and TNF-α. In contrast, classic hormones, such as insulin, are secreted from discrete glands (e.g., the pancreas). As of 2008, the current terminology refers to cytokines as immunomodulating agents. However, more research is needed in this area of defining cytokines and hormones.
Part of the difficulty with distinguishing cytokines from hormones is that some of the immunomodulating effects of cytokines are systemic rather than local. For instance, to use hormone terminology, the action of cytokines may be autocrine or paracrine in chemotaxis or chemokinesis and endocrine as a pyrogen. Further, as molecules, cytokines are not limited to their immunomodulatory role.
Cytokines have been classed as lymphokines, interleukins, and chemokines, based on their presumed function, cell of secretion, or target of action. Because cytokines are characterised by considerable redundancy and pleiotropism, such distinctions, allowing for exceptions, are obsolete.
The term interleukin was initially used by researchers for those cytokines whose presumed targets are principally leukocytes. It is now used largely for designation of newer cytokine molecules discovered every day and bears little relation to their presumed function. The vast majority of these are produced by T-helper cells.
The first of these three, the IL-2 subfamily, is the largest. It contains several non-immunological cytokines including erythropoietin (EPO) and thrombopoietin (TPO). Furthermore, four-α-helix bundle cytokines can be grouped into long-chain and short-chain cytokines.
the IL-1 family, which primarily includes IL-1 and IL-18
the IL-17 family, which has yet to be completely characterized, though member cytokines have a specific effect in promoting proliferation of T-cells that cause cytotoxic effects.
A classification that proves more useful in clinical and experimental practice outside of structural biology divides immunological cytokines into those that enhance cellular immune responses, type 1 (INf-γ, TNFα, etc.), and type 2 (TGF-β, IL-4, IL-10, IL-13, etc.), which favor antibody responses.
A key focus of interest has been that cytokines in one of these two sub-sets tend to inhibit the effects of those in the other. Dysregulation of this tendency is under intensive study for its possible role in the pathogenesis of autoimmune disorders.
Several inflammatory cytokines are induced by oxidant stress. The fact that cytokines themselves trigger the release of other cytokines  and also lead to increased oxidant stress makes them important in chronic inflammation, as well as other immunoresponses, such as fever and acute phase proteins of the liver (IL-1,6,12, INF-a).
In recent years, the cytokine receptors have come to demand the attention of more investigators than cytokines themselves, partly because of their remarkable characteristics, and partly because a deficiency of cytokine receptors has now been directly linked to certain debilitating immunodeficiency states. In this regard, and also because the redundancy and pleiomorphism of cytokines are, in fact, a consequence of their homologous receptors, many authorities think that a classification of cytokine receptors would be more clinically and experimentally useful.
A classification of cytokine receptors based on their three-dimensional structure has, therefore, been attempted. Such a classification, though seemingly cumbersome, provides several unique perspectives for attractive pharmacotherapeutic targets.
Hemopoietic Growth Factor (type 1) family, whose members have certain conserved motifs in their extracellular amino-acid domain. The IL-2 receptor belongs to this chain, whose γ-chain (common to several other cytokines) deficiency is directly responsible for the x-linked form of Severe Combined Immunodeficiency (X-SCID).
Interferon (type 2) family, whose members are receptors for IFN β and γ.
Tumor necrosis factors (TNF) (type 3) family, whose members share a cysteine-rich common extracellular binding domain, and includes several other non-cytokine ligands like CD40, CD27 and CD30, besides the ligands on which the family is named (TNF).
Interleukin-17 receptor (IL-17R) family, which shows little homology with any other cytokine receptor family. Structural motifs conserved between members of this family include: an extracellular fibronectin III-like domain, a transmembrane domain and a cytoplasmic SERIF domain. The known members of this family are as follows: IL-17RA, IL-17RB, IL-17RC, IL17RD and IL-17RE.
Interleukin-12 (IL-12), secreted by macrophages and act on TH0 (T-helper cell 0) and convert into TH1 (T-helper cell 1)which produce IFN γ. IFN γ activates macrophages, thus starting cell mediated immune response.
Each cytokine has a matching cell-surface receptor. Subsequent cascades of intracellular signalling then alter cell functions. This may include the upregulation and/or downregulation of several genes and their transcription factors, resulting in the production of other cytokines, an increase in the number of surface receptors for other molecules, or the suppression of their own effect by feedback inhibition.
The effect of a particular cytokine on a given cell depends on the cytokine, its extracellular abundance, the presence and abundance of the complementary receptor on the cell surface, and downstream signals activated by receptor binding; these last two factors can vary by cell type. Cytokines are characterized by considerable "redundancy", in that many cytokines appear to share similar functions.
It seems to be a paradox that cytokines binding to antibodies have a stronger immune effect than the cytokine alone. This may lead to lower therapeutic doses.
Said et al. showed that inflammatory cytokines cause an IL-10-dependent inhibition of  T-cell expansion and function by up-regulating PD-1 levels on monocytes which leads to IL-10 production by monocytes after binding of PD-1 by PD-L.
Adverse reactions to cytokines are characterized by local inflammation and/or ulceration at the injection sites. Occasionally such reactions are seen with more widespread papular eruptions.
Roles of endogenous cytokines in health and disease
Cytokines are crucial for fighting off infections and in other immune responses. However, they can become dysregulated and pathological in inflammation, trauma, and sepsis.
Adverse effects of cytokines have been linked to many disease states and conditions ranging from major depression and Alzheimer's disease to cancer with levels either being elevated or changed. Over-secretion of cytokines can trigger a dangerous syndrome known as a cytokine storm; this may have been the cause of severe adverse events during a clinical trial of TGN1412. Cytokine storms also were the main cause of death in the 1918 "Spanish Flu" pandemic. Deaths were weighted more heavily towards people with healthy immune systems, due to its ability to produce stronger immune responses, likely increasing cytokine levels. Another important example of cytokine storm is seen in acute pancreatitis. Cytokines are integral and implicated in all angles of the cascade resulting in the systemic inflammatory response syndrome and multi organ failure associated with this intra-abdominal catastrophe.
Research into diagnostic use of measured levels
Plasma levels of various cytokines may give information on the presence, or even predictive value of inflammatory processes involved in autoimmune diseases such as rheumatoid arthritis, as well as immunomodulatory effects of foods or drugs. In addition, elevated levels of IL-7, an important cytokine involved in T cell homeostasis, have been detected in the plasma of HIV-infected patients.
^Saito explains "much evidence has suggested that cytokines and chemokines play a very important role in the reproduction, i.e. embryo implantation, endometrial development, and trophoblast growth and differentiation by modulating the immune and endocrine systems."(15)
^Chen explains the regulatory activity of LIF in human and murine embryos: "In conclusion, human preimplantation embryos express LIF and LIF-R mRNA. The expression of these transcripts indicates that preimplantation embryos may be responsive to LIF originating either from the surrounding environment or from the embryos themselves and exerting its function in a paracrine or autocrine manner."(719)
^"Cytokine" in John Lackie. A Dictionary of Biomedicine. Oxford University Press. 2010. ISBN 9780199549351
^“Cytokine” in Stedman’s Medical Dictionary, 28th ed. Wolters Kluwer Health, Lippincott, Williams & Wilkins (2006)
^ abHorst Ibelgaufts. Cytokines in Cytokines & Cells Online Pathfinder Encyclopedia Version 31.4 (Spring/Summer 2013 Edition)
^Vlahopoulos S, Boldogh I, Casola A, Brasier AR (September 1999). "Nuclear factor-kappaB-dependent induction of interleukin-8 gene expression by tumor necrosis factor alpha: evidence for an antioxidant sensitive activating pathway distinct from nuclear translocation". Blood94 (6): 1878–89. PMID10477716.
^Chokkalingam, V.; Tel, J.; Wimmers, F.; Liu, X.; Semenov, S.; Thiele, J.; Figdor, C. G.; Huck, W. T. S. (2013). "Probing cellular heterogeneity in cytokine-secreting immune cells using droplet-based microfluidics". Lab on a Chip13 (24): 4740–4744. doi:10.1039/C3LC50945A. PMID24185478.edit
^Horst Ibelgaufts. Recombinant cytokines in Cytokines & Cells Online Pathfinder Encyclopedia Version 31.4 (Spring/Summer 2013 Edition)
^Dimiter S. Dimitrov. Therapeutic Proteins.Chapter 1 in Therapeutic Proteins: Methods and Protocols, Editors: Vladimir Voynov, Justin A. Caravella. Volume 899 of Methods in Molecular Biology. Springer Science+Business Media, LLC 2012. ISBN 978-1-61779-920-4 (Print) 978-1-61779-921-1 (Online)