Newly synthesised mediators
The interaction of granule-containing immune cells with pathogens not only results in the immediate large scale release of pre-stored mediators through degranulation but also in the secretion of newly synthesised mediators through smaller vesicles. As these mediators have to be made, their release is delayed and occurs minutes to hours after the initial pathogen encounter. Lipid-derived mediators, like platelet activating factor, leukotrienes and prostaglandins belong to this group. Leukotrienes and prostaglandins are generated and released within minutes of cell activation. Both dilate blood vessels which slow the blood flow down and thereby help the recruitment of inflammatory cells from the blood into the tissue. Leukotrienes then increase the activity of the infiltrating granulocytes to make them more effective killers and enhance the phagocytic activity of macrophages already in the tissue which clear dead and dying cells. Prostaglandins also cause pain and fever and the increased body temperature inhibits proliferation of many pathogens. Prostaglandins further promote blood clotting by acting on platelets which then act like a net capturing pathogens. Again all these functions making the environment as inhospitable as possible for the pathogen. Levels of both leukotrienes and prostaglandins are elevated in allergy diseases.
Other newly synthesised mediators released with a time delay include chemokines and pro-inflammatory cytokines. Find out what they do on the next page!