Mast cells (MCs) were first described by Paul Ehrlich in his doctoral thesis (Ehrlich, 1878). He discovered these cells in various organs of several animal species andrecognized their tendency to concentrate around blood vessels, nerves and glandular ducts as well as in inflammatory and neoplastic foci'(Selye, 1965). MCs are derived from precursors of the haematopoietic lineage and complete their differentiation in peripheral tissues (Galli, 1990). All mammalian MCs express common characteristics, including plasma membrane receptors binding IgE antibodies and cytoplasmic granules storing biogenic amines, proteoglycans, cytokines and neutral proteases (Figs 1 and 2). However, MC populations show marked differences in their phenotypic expression in distinct anatomical sites, a phenomenon calledmast cell heterogeneity'(Galli, 1990). Human MCs are conventionally divided into two types depending on the expression of different proteases in their granules. MCT cells (also regarded as immune cell associated) contain tryptase and are predominantly located in the respiratory and intestinal mucosa, where they colocalize around T lymphocytes. MCTC cells contain both tryptase and chymase and are predominantly found in connective tissue areas, such as skin, conjunctiva and synovium. Typically, MCs stain metachromatically, because of the presence of the sulphated glycosaminoglycan heparin.