Human mast cell tryptases comprise a family of trypsin-like neutral serine proteases that are predominantly expressed in mast cells. In its enzymatically active form, mast cell tryptase exists as a non-covalently linked tetramer of 132 kDa. Mast cell tryptase is capable of degrading vasoactive intestinal peptide and activating prekallikrein as well as generating kinins, all important mediators involved in bronchoconstriction and airway hyperresponsiveness, which are major contributors to allergic airway disease. Mast cells are activated by a number of stimuli, including antigen, superoxides, complement protein, neuropeptides and lipoproteins, resulting in activation and degranulation. Mast cells play an active role in such diverse diseases as atherosclerosis, asthma, arthritis, bile duct fibrosis, malignancy and pulmonary fibrosis. Identification of mast cells through staining of tissues with antibodies specific for human mast cell tryptase has been useful in identification of focal and diffuse MC infiltrates in primary MC disease and mastocytosis. Demonstration of mast cell tryptase is essential for the identification of highly atypical, hypogranulated or even non-metachromatic MCs, especially in MC leukemia, and for the detection of small or minute MC infiltrates