Leukotriene C4 (LTC4) is a cysteinyl leukotriene (CysLT), a family of potent inflammatory mediators. Eosinophils, one of the principal cell types recruited to and activated at sites of allergic inflammation, is capable of elaborating lipid mediators, including leukotrienes derived from the oxidative metabolism of arachidonic acid (AA). Potentially activated eosinophils may elaborate greater quantities of LTC4, than normal eosinophils. These activated eosinophils thus are primed for enhanced LTC4 generation in response to subsequent stimuli. Some recognized priming stimuli are chemoattractants (e.g. eotaxin, PAF) that may participate in the recruitment of eosinophils to sites of allergic inflammation. The mechanisms by which chemoattractants and other activating cytokines (e.g. interleukin (IL)-5) or extracellular matrix components (e.g. fibronectin) enhance eosinophil eicosanoid formation are pertinent to the functions of these eicosanoids as paracrine mediators of allergic inflammation. Some eosinophil-derived eicosanoids may be active in down-regulating inflammation. It is increasingly likely that eicosanoids synthesized within cells, including eosinophils, may have intracellular (e.g. intracrine) roles in regulating cell functions, in addition to the more recognized activities of eicosanoids as paracrine mediators of inflammation. Acting extracellularly, the cysteinyl leukotrienes (CysLTs) LTC4 and its extracellular derivatives, LTD4 and LTE4 are key paracrine mediators pertinent to asthma and allergic diseases. Based on their receptor-mediated capabilities, they can elicit bronchoconstriction, mucus hypersecretion, bronchial hyperresponsiveness, increased microvascular permeability, and additional eosinophil infiltration. Eosinophils are a major source of CysLTs and have been identified as the principal LTC4 synthase expressing cells in bronchial mucosal biopsies of asthmatic subjects (PMID: 12895596). Leukotrienes are eicosanoids. The eicosanoids consist of the prostaglandins (PGs), thromboxanes (TXs), leukotrienes (LTs), and lipoxins (LXs). The PGs and TXs are collectively identified as prostanoids. Prostaglandins were originally shown to be synthesized in the prostate gland, thromboxanes from platelets (thrombocytes), and leukotrienes from leukocytes, hence the derivation of their names. All mammalian cells except erythrocytes synthesize eicosanoids. These molecules are extremely potent, able to cause profound physiological effects at very dilute concentrations. All eicosanoids function locally at the site of synthesis, through receptor-mediated G-protein linked signalling pathways.