Mast cells are not seen in routine histological sections stained with hema toxilin and eosin, but a considerable number of mast cells are found in various tissues that are properly fixed and stained with dyes such as toluidine blue and Alcian blue. The substances in granules that stain specifically with these dyes are proteoglycans, which are negatively charged and thought to form complexes with positively charged proteases and histamine.

Mast cells have high affinity IgE receptors on their surface, and the immunological activity of mast cells is mediated through these IgE recep tors (1). Binding of antigens to IgE molecules results in the formation of linkages between IgE receptors, and then the release of the granules themselves or chemical mediators in the granules (2). This process con stitutes an important step in the immediate hypersensitivity reaction that occurs in allergic diseases such as urticaria, bronchial asthma, and allergic rhinitis. In addition to having a role in allergic diseases, mast cells have a physiological role as an effector of host defense mechanisms in intestinal helminth infection (3-5) and dermal tick infestation (6, 7). Although some of cytochemical and functional characteristics of mast cells are common also to basophils, these two types of cells can be dis tinguished with an electron microscope (8). Moreover, their differentiation