||Electron microscopy. This mucosal mast cell of the lung is localized in the vicinity of a blood vessel. Notice the smooth muscle cells (1) of a small arteriole and collagen fibers (2). Most obvious is the presence of granules varying in shape and size. These membrane-bound vesicles (so-called compound granules) show a metachromatic reaction in light microscopy and ultra structurally a granule exhibits a heterogeneous content (different with species) e.g. osmiophilic granular, filamentous, whorl-like substances. These structures contain among others heparin, histamine, enzymes such as tryptase, (superoxide dismutase, beta-hexosaminidase) factors such as neutrophil- and eosinophil-chemotactic factors, vasoactive mediators. The content of the granules varies strongly with different species (rodent, human). A second subset of mast cells is the connective tissue mast cell. In rodents the major granule proteoglycan is heparin (and histamine). Mast cells are activated by cross-linking of the IgE-receptors occupied by IgE and by allergen. Activation of mast cells results in three types of response: (1) secretion of preformed contents of their granules (exocytosis); (2) synthesis and secretion of lipid mediators; (3) synthesis and secretion of cytokines. Eventually the degranulation can result in an immediate hypersensitivity (anaphylactoid) reaction.