Mast cells are found resident in tissues throughout the body, particularly in 
association with structures such as blood vessels and nerves, and in proximity 
to surfaces that interface the external environment. Mast cells are bone 
marrow-derived and particularly depend upon stem cell factor for their survival. 
Mast cells express a variety of phenotypic features within tissues as determined 
by the local environment. Withdrawal of required growth factors results in mast 
cell apoptosis. Mast cells appear to be highly engineered cells with multiple 
critical biological functions. They may be activated by a number of stimuli that 
are both Fc epsilon RI dependent and Fc epsilon RI independent. Activation 
through various receptors leads to distinct signaling pathways. After 
activation, mast cells may immediately extrude granule-associated mediators and 
generate lipid-derived substances that induce immediate allergic inflammation. 
Mast cell activation may also be followed by the synthesis of chemokines and 
cytokines. Cytokine and chemokine secretion, which occurs hours later, may 
contribute to chronic inflammation. Biological functions of mast cells appear to 
include a role in innate immunity, involvement in host defense mechanisms 
against parasitic infestations, immunomodulation of the immune system, and 
tissue repair and angiogenesis.