Most physicians and immunologists, if they think of the mast cell at all, regard it as something of a pariah. Other cells of hematopoietic origin, such as neutrophils, macrophages, and platelets, are clearly important in host defense, hemostasis, or both. In contrast, the contributions of the mast cell to pathologic conditions come to mind much more readily than any role it may have in the maintenance of health. Indeed, some would argue that an entire branch of medicine -- allergy -- deals in large part with the adverse effects produced by activation of the mast cell and the basophil, a . . .