ABSTRACT
Mast cells and basophils are generally recognized as the principal cell types to initiate IgE-dependent, type I immediate hypersensitivity reactions. More recently, mast cells have been implicated as participants in innate immunity, additional aspects of acquired immunity, and tissue remodeling (Fig. 1). Mast cells originate from bone marrow progenitors but complete their maturation in tissues where they then reside. Basophils also originate from bone marrow progenitors but complete their maturation before being released from the bone marrow into the circulation, where they reside until called into tissues at sites of inflammation, particularly during the late phase of IgE-mediated immediate hypersensitivity reactions and during the early phase of cell-mediated, delayed-type hypersensitivity reactions. Mast cells and basophils are the only two cell types that constitutively express substantial quantities of the high-affinity, tetrameric receptor for IgE and store histamine in their secretory granules. These two cell types are distinguished from one another by their respective pathways for growth, differentiation, and survival; patterns of cell-surface adhesion, cytokine and chemokine receptors; responses to non-IgE-dependent agonists; secretory granule proteoglycans and proteases; and morphologies. For example, nuclei of peripheral blood basophils have deeply divided lobes, whereas those of mast cells in normal tissues are rounded.
