An object or substance by which something is mediated, such as :

1. A structure of the nervous system that transmits impulses eliciting a specific response.
2. A chemical substance (transmitter substance) that induces activity in an excitable tissue, such as nerve or muscle (e.g. hormones ).
3. A substance released from cells as the result of an antigen-antibody interaction or by the action of antigen with a sensitized lymphocyte (e.g. cytokine ).

Concerning mediators of Immediate Hypersensitivity , the most important include Histamine , Leukotriene (e.g. SRS-A ), ECF-A , PAF and Serotonin . There also exist three classes of lipid mediators whcih are synthesized by activated mast cells through reactions initiated by the actions of phospholipase A2 . These are Prostaglandins , Leukotrienes and Platelet-activating factors (PAF).