Mast cells are the primary effector cell type in urticaria and angioedema. Recognition of different types of mast cells has increased the understanding of their cell biology and may help refine the therapy of human allergic diseases. Mast cells containing chymase and tryptase (MCTC) and tryptase alone (MCT) are two distinct types distinguished on the basis of the neutral protease composition of their granules. MCT cells are distributed primarily in the lung and gastrointestinal mucosa, whereas MCTC cells lie primarily in skin and gastrointestinal submucosa. The appearance of MCT cells in intestinal tissue is T-lymphocyte dependent, whereas MCTC cells is not. The granules in unstimulated mature MCT cells typically contain complete scrolls, whereas those of MCTC cells often contain grating or lattice substructures. Major categories for the mediators of mast cells include performed mediators present in the secretory granule, newly generated lipid-derived mediators, and cytokines.