Low molecular weight protein mediators involved in cell growth, inflammation, immunity, differentiation and repair are now generically termed 'cytokines'. There have been many previous attempts at a suitable nomenclature, but 'cytokines' is the current most widely used compromise. Previous attempts include 'lymphokines', 'monokines' and 'interleu-kins'. 'Lymphokines' dates from the 1970s, for products of lymphocytes, designed to contrast with 'monokines' for products of monocytes. However, it is now known that there is considerable overlap -for example, tumor necrosis factor a (TNFa), interleukin 6 (IL-6) are made by both cell types - so this nomenclature is no longer in wide use.

'Interleukins' describes molecular messengers acting between leukocytes. The term dates from 1979, at a conference in Switzerland where it became apparent that a variety of biological activities termed 'lymphocyte-activating factor', 'endogenous pyrogen', 'catabolin' and so on, were due to molecules with similar biochemical characteristics, and the term 'interleukin 1' was adopted. Currently there are 17 molecules with a wide spectrum of activities, designated interleukins 1-17. However, there are deficiencies in this nomenclature; it is becoming abundantly clear that these molecules are often made by nonblood cells, and act on cells apart from blood cells, e.g. IL-6, IL-1.

Two other related groups of molecules have been described: 'interferons' and 'growth factors'. 'Interferons' are molecules first described in the 1960s as protein, with nonspecific anti-viral effects. These are now known to have many other effects; thus interferon y (IFNy) is one of the most potent immu-noregulatory mediators known; all interferons have immunoregulatory and cytostatic as well as antiviral actions. 'Growth factors' are molecules which stimulate the growth of a variety of cells. Some are more specific than others, for example granulocyte-macrophage colony-stimulating factor (GM-CSF) acts on monocytes/macrophages and granulocyte cells to promote their growth, while platelet-derived growth factor (PDGF), which is three related molecules, acts on most connective tissue cells. Hematopoietic growth factors, also termed 'colony-stimulating factors' from their initial bioassays, are, in common with the other molecules in this category, not restricted to mediating growth. They are all also involved in regulating cellular activation. Thus 'growth factors' is not an accurate terminology either.

There is a large family of chemotactic cytokines of low molecular weight (8-10 kDa) which are now termed 'chemokines'. These include IL-8, MCP-1, etc, and are grouped into two major families, depending on the position of cysteine, as C-C or C-X-C chemokines. The latter act chiefly on granulocytes, the former on monocytes and lymphocytes.

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