Mediators of inflammatory reactions

Mediators of inflammatory reactions: Histamine, kinins, acute-phase proteins, and defensin are the important mediators of inflammatory reactions.

·           Histamine: It is a chemical substance produced by a varietyof cells in response to tissue injury. It is one of the principal mediators of the inflammatory response. It binds to recep-tors on nearby capillaries and venules, causing vasodilata-tion and increased permeability.

·           Kinins: These are other important mediators of inflamma-tory response. They are normally present in blood plasma in an inactive form. Tissue injury activates these small peptides, which then cause vasodilatation and increased permeability of capillaries. Bradykinin also stimulates pain receptors in the skin. This effect probably serves a protective role because pain normally causes an individual to protect the injured area.

·           Acute-phase proteins: These include C-reactive pro-teins and mannose-binding proteins that form part of the innate immunity. These proteins are produced at an increased concentration in plasma during acute-phase reaction, as a nonspecific response to microorganisms and other forms of tissue injury. They are synthesized in the liver in response to cytokines called proinflammatorycytokines, namely, interleukin-1 (IL-1), interleukin-6 (IL-6), and tissue necrosis factor (TNF). They are called pro-inflammatory cytokines because they enhance the inflam-matory responses.

·           Defensins: They are another important component of theinnate immunity. They are cationic peptides that produce pores in membrane of the bacteria and thereby kill them. These are present mainly in the lower respiratory tract and gastrointestinal tract. The respiratory tract contains b-defensins, whereas the gastrointestinal tract contains a-defensins. The a-defensins also exhibit antiviral activ-ity. They bind to the CXCR4 receptors and block entry of HIV virus into the cell. How these defensins differentiate microbes from some cells is not known.