Mast cells, historically known for their involvement in type I hypersensitivity, also serve critical protective and homeostatic functions. They directly recognize the products of bacterial infection through several surface receptor proteins, releasing proteases, cytokines, and eicosanoid mediators that recruit neutrophils, limit the spread of bacterial infection, and facilitate subsequent tissue repair. In vitro studies suggest that the spectrum of microbes capable of initiating mast cell activation is broad and extends to common respiratory viruses, mycoplasma, and even products of tissue injury, such as nucleotides. TH2-polarized inflammation elicits a reactive hyperplasia of mast cells at the involved mucosal surfaces in both mice and human subject. Several recombinant TH2 cytokines (IL-3, IL-4, IL-5, and IL-9) act synergistically with stem cell factor to facilitate proliferation of nontransformed human mast cells in vitro. IL-4 induces the expression of critical inflammation-associated genes by human mast cells, such as those encoding leukotriene C4 synthase, Fc(epsilon)RI, and several cytokines. Consequently, priming with IL-4 not only amplifies classical Fc(epsilon)RI-dependent mast cell activation but also dramatically alters the product profile of mast cells activated by innate signals and by chemical mediators of inflammation. Strikingly, IL-4 induces an activation response by mast cells to cysteinyl leukotrienes, which act through a receptor shared with uridine diphosphate to induce cytokine generation without exocytosis. It Is possible that alterations in mast cell phenotype by the TH2 milieu of allergy permits otherwise trivial infections or homeostatic chemical signals to initiate harmful inflammatory cascades and sustain tissue pathology. Drug development must take these nonclassical mast cell activation pathways into account without compromising the beneficial and protective functions of mast cells.