The mast cell is the cellular basis for immediate hypersensitivity reactions. 
The specificity of the immediate hypersensitivity reaction is attributable to 
IgE molecules fixed to specific membrane receptors which, when stimulated by 
specific antigen, initiates the process of degranulation of the mast cell. The 
granules provide three separate sources of biologic activity: performed or 
primary mediators, newly generated or secondary mediators, and activities 
associated with the granular matrix. A number of biologic consequences are 
generated in response to these mediators and these include: increased vascular 
permeability, vasodilation, smooth muscle spasm, polymorphonuclear leukocyte 
chemotaxis, stimulation of adenylate and guanylate cyclase, superoxide radical 
generation, prostaglandin formation, mucous and gastric acid secretion, 
hypotension, tissue destruction, and mononuclear leukocyte infiltration. This 
pharmacopia of activities accounts for the clinical aspects of allergic 
diseases, suggests that the mast cell granule may be involved in the host's 
defense against parasitic infections, and is compatible with a suggested role of 
the mast cell as a widely distributed, monocellular endocrine system.