Mast cells are important effector cells of the immune system and recent studies 
show that they have immunomodulatory roles in diverse processes in both health 
and disease. Mast cells are distinguished by their high content of 
electron-dense secretory granules, which are filled with large amounts of 
preformed and pre-activated immunomodulatory compounds. When appropriately 
activated, mast cells undergo degranulation, a process by which these preformed 
granule compounds are rapidly released into the surroundings. In many cases, the 
effects that mast cells have on an immune response are closely associated with 
the biological actions of the granule compounds that they release, as 
exemplified by the recent studies showing that mast cell granule proteases 
account for many of the protective and detrimental effects of mast cells in 
various inflammatory settings. In this Review, we discuss the current knowledge 
of mast cell secretory granules.