Mast cell biology.

Mast cells are the repository for histamine in the body. They influence the pathophysiology of allergic diseases, such as rhinitis, urticaria, and asthma; regulate bone formation and integrity; help repair and maintain connective tissue; promote wound healing; and probably contribute to the development and preservation of the endothelium and small blood vessels. Although they are found in all human tissue, mast cells are most prevalent at the interface between the host and its environment, that is, in the skin and in the mucosa of the upper and lower respiratory tracts and the gastrointestinal tract. Recent evidence suggests that two types of mast cells exist: (1) the connective tissue type, found primarily but not exclusively in loose connective tissue and skin, and (2) the mucosal type, found primarily in gastrointestinal mucosa and peripheral airways. The factors that produce this differentiation are not fully known. Although both mast cell types have IgE receptors that can be activated by allergens, differences between the two types exist in their responses to nonallergic signals, the mediators they release, their proteoglycan constituents, and the makeup of their granular enzymes. The importance of these biochemical differences to cellular functioning remains to be investigated.