Cytokines

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In medicine, cytokines are the primary intercellular chemical messengers of the immune system. Chemically, they are water-soluble proteins and glycoproteins with a mass of 8 to 30 kDaltons (kDa). These protein messengers are produced and released by cells of the immune system such as B-lymphocytes, macrophages and T-lymphocytes. Their actions are essential to the the activation and control of immune responses and the development of blood cells. Once they attach to the surface of a target cell, they may invoke a second messenger system, which causes a release, inside the cell, of chemical messenger(s) that cause specific activities. Those activities may include the extracellular release of additional, usually differentiated cytokines, so cells amplify cytokines. Mammalian cytokines are "non-antibody proteins secreted by inflammatory leukocytes and some non-leukocytic cells, that act as intercellular mediators. They differ from classical hormones in that they are produced by a number of tissue or cell types rather than by specialized glands. They generally act locally in a paracrine or autocrine rather than endocrine manner."^[1] Examples include growth factors,interferons, interleukins, and tumor necrosis factor.

Plants may synthesize cytokines, which affect the plants, but some, such as abscisic acid, also affect humans.^[2] In botany, however, the cytokinins are a group distinct from animal cytokines.

Cytokine is not the universal name for the group.^[3] Other name refer to sources of specific kinds of cytokines, or their categorizing by target of action. Once the amino acid sequence of a cytokine is known, by convention, it is reclassified as an interleukin. ^[4]

Categorizing by source[edit]

They may be named by their source:

• lymphokine (cytokines made by lymphocytes)
• monokine (cytokines made by monocytes)
• chemokine (cytokines with chemotactic activities)
• interleukin (cytokines made by one leukocyte and acting on other leukocytes).

Categorizing by target[edit]

They also may be categorized by their target; some cytokines have more than one of the type:

• On the secreting cell: autocrine cytokine
• on nearby cells: paracrine
• on distant cells: endocrine action. This last group overlaps releasing factors and hormones.

Categorizing by structure[edit]

Structurally, cytokines can be classified into several classes: ^[5]

• four alpha-helix bundle family consisting of the IL-2 subfamily (including erythropoietin and thrombopoietin,
• interferon subfamily (approximately 20 α-interferon and 1 β-interferon in Type I, one β-interferon in Type II) and the IL-10 subfamily^[6]
• IL-1 family (primarily IL-1 and IL-18)
• IL-17 family
• chemokine family

Categorizing by receptor[edit]

• Immunoglobulin (Ig-x) superfamily are found throughout the body (eg, IL-1 receptor types)
• Interferon (type 2) family includes IFN beta and gamma receptors.
• Tumor necrosis factor (TNF) (type 3) family is composed of receptors sharing a cystein-rich extracellular binding domain and includes non-cytokine ligands such as CD40, CD27, and CD30 in addition to TNF.
• 7-transmembrane helix family that includes all G-protein coupled receptors (GPCRs), such as the chemokine receptors CXCR4 and CCR5 (HIV binding proteins) belong to this family.

Categorizing by hybrid, functional organization[edit]

Major chemical families include:

• Interleukins (IL) (formally called lymphocyte activating factor)
• Interferons (INF)
• Tumor necrosis factors (TNF)
• Granulocyte-Monocyte Colony Stimulating Factor
• Erythrocyte Stimulating Factor
• Macrophage chemotactic protein (MCP)
• Macrophage inflammatory protein (MIP)

References[edit]