Mast cells are one of the major effector cells in the pathogenesis of the 
immediate-type hypersensitivity reaction in a number of non-allergic immune 
disorders as well as in normal physiological processes. In addition, it has been 
shown recently that mast cells also play a significant role in a life-saving 
host response to bacterial reactions. But as much as the immunopathological role 
of mast cells has been acknowledged, these cells have also aroused much 
controversy and confusion. By now it is clear that one explanation for the 
sometimes even contradictory opinions on mast cell function arise from mast cell 
heterogeneity. This heterogeneity can express itself as differences in 
histochemical, biochemical, and functional characteristics. In vitro systems 
provided a powerful tool for the investigation of the basic mechanisms for mast 
cell development and differentiation and helped to demonstrate that mast cell 
heterogeneity can be traced back to certain cytokine patterns that are present 
in different microenvironments. In this context it has also been shown that the 
growth factors required for human mast cell differentiation are somewhat 
different than those for rodents. In rodents, the atypical, T cell-dependent 
mucosal type mast cell can be distinguished from the T cell-independent 
connective tissue-type mast cell. In humans, the strict classification into 
mucosal and connective tissue-type mast cells is not possible and the content of 
mast cell-specific proteases chymase and tryptase is the main criterion for mast 
cell subtypes in humans. The large quantities of tryptase and chymase that are 
synthesized by mast cells suggest and emphasize the significance of these 
proteinases in mast cell function and stimulated investigations about the 
biological properties of these mast cell-specific proteases. Comparing their 
biological activities it becomes clear that they share some activities. On the 
other hand, tryptase seems to participate in proinflammatory mast cell function, 
whereas chymase seems to be more involved in inflammatory reactions. This review 
provides a short overview of the discovery, origin, development, and biological 
significance of mast cells and will then concentrate on mast cell heterogeneity 
in rodents and humans with respect to the mast cell proteases tryptase and 
chymase and their function.