Exploring the mast cell enigma: a personal reflection of what remains to be done.

Mast cells are traditionally viewed as effector cells of allergic reactions and parasitic diseases, but their importance in host defense against bacteria, in tissue remodelling, their bone marrow and stem cell origin and a central role of the stem cell factor (SCF) as mast cell growth and chemotactic factor has been worked out only in recent years. Despite this, major aspects about the nature of the cells and their role in disease remain unclear. This holds in particular for the identification of mast cell precursors and the role of growth factors that stimulate specific mast cell commitment from stem cells, such as nerve growth factor, neutrotrophin-3 and certain interleukins, alone and during interaction with SCF. Early data suggesting also an involvement of specific transcription factors need to be expanded in this process. Furthermore, although mast cell proliferative disease (mastocytosis) has been shown to be often associated with SCF receptor c-kit mutations, reasons for the development of this disease remain unclear. This holds also for mast cell release mechanisms in many types of mast cell-dependent urticaria. Exciting new insights are emerging regarding the role of mast cells in bacterial infections, in defense against tumors, in wound healing and in the interplay with the nervous system, with hormones, and in the neurohormonal network. The aim of this reflection is to delineate the many known and unknown aspects of mast cells, with a special focus on their development, and to discuss in detail two mast cell-related diseases, namely mastocytosis and urticaria.