Mast cells can be defined as resident tissue cells bearing high-affinity binding sites for IgE, synthesizing and storing histamine and proteoglycans within cytoplasmatic granules, the latter being responsible for the metachromatic staining. Within parenchymatous organs, mast cells are distributed only in moderate numbers; however, they are found with high density in alveolar walls in nasal, conjunctival and intestinal mucosa, and in the skin, depending on the anatomic site and the age. In rodents, subpopulations in skin and intestinal sub-mucosa can be defined as connective tissue mast cells, or in mucosa of lung and gut as mucosa mast cells, respectively.