I. Introduction There has long been evidence supporting the idea that mast cells may play a key role in allergic disease of the respiratory tract. This cell type is ubiquitous in its distribution, but it is particularly prevalent at the major portals of allergen entry to the body. In the lungs, mast cells are present in large numbers in the bronchial mucosa and submucosa (1) and the alveolar walls (2), and they may be sited in the epithelial lining and even free in the bronchial lumen (3,4). At these locations, inhaled allergen can bind readily to allergen-specific immunoglobulin E (lgE) on high-affinity lgE receptors, triggering the explosive release of potent mediators of inflammation and bronchoconstriction.