This vast group are molecules produced for inter- and intracellular signalling.
They are intimately involved in the inflammatory and immune responses as both effectors and a means of co-ordinating the level of activity. Their interaction is undoubtedly complex and poorly understood. It is important to emphasise that in vitro activity does not necessarily equate with a role in vivo.
One means of classifying them is by tissue of origin:
- plasma-derived: inactive precursors produced in liver and circulate as plasma proteins. Activated by binding specific substance which then trigger proteolysis of the next plasma protein in sequence. Examples include:
- kinin system
- clotting system
- fibrinolytic system
- complement cascade
- tissue-derived: a diverse range of substances whose release is triggered by a number of stimuli e.g. direct injury, neurogenic stimuli, immune mechanisms or other inflammatory mediators. Examples include:
- vasoactive amines: histamine and serotonin
- eicosanoids - arachidonic acid metabolites
- acute phase proteins
- platelet activating factor
- lysosomal constituents
- growth factors