Mast cells, basophils, and eosinophils have long been regarded as important effector cells in allergic disorders. Indeed, it is thought that the cells' cytoplasmic granule-associated or lipid mediators contribute to many of the signs and symptoms that are characteristic of these diseases. Mast cells, basophils, and eosinophils also probably contribute to protective host responses, especially to parasites. In addition, recent evidence shows that mast cells, basophils, and eosinophils can secrete a wide spectrum of cytokines and, in some cases, express functions that may permit them to regulate the development or perpetuation of allergic responses. Thus, mast cells, basophils, and eosinophils may express immunoregulatory activities, as well as serve as effector cells.