The mast cell: an active participant or an innocent bystander?

Mast cells (MC) are phylogenetically old cells which are distributed throughout the human organism and, on the whole, occupy roughly the volume of the spleen. MC have long been recognized as key cells of type I hypersensitivity reactions. Several lines of evidence, however, indicate that they not only express critical effector functions in classic IgE-associated allergic disorders, but also play important roles in host defence against parasites, bacteria and perhaps even viruses. Indeed, it is now clear that MC can contribute to host defence in the context of either acquired or innate immune responses through the release of a myriad of pro-inflammatory and immunoregulatory molecules and the expression of a wide spectrum of surface receptors for cytokines and chemokines. Moreover, there is growing evidence that MC exert distinct nonimmunological functions, playing a relevant role in tissue homeostasis, remodeling and fibrosis as well as in the processes of tissue angiogenesis. In this review, we provide a small insight into the biology of human MC and their potential implications in clinical pathology.