Nonredundant Roles for Basophils in Immediate- and Delayed-onset Allergic Reactions

Basophils circulate in the peripheral blood under physiological conditions, and they are recruited to affected tissues in allergic reactions, albeit in small numbers. Because of their rarity (less than 1% of peripheral blood leukocytes are basophils), and their similarity to mast cells, basophils have often been considered the lesser relatives of mast cells. Moreover, because basophils have been so difficult to identify, mice were erroneously believed for a long time to lack them. Therefore, the assumption that basophils have only redundant roles has remained unquestioned until recently. The flow-cytometric identification of basophils in mice and the development of in vivo models and reagents useful for their functional analyses have greatly advanced the field of basophil research. Previously unrecognized roles of basophils, distinct from those of mast cells, have been shown in allergic responses and the regulation of acquired immunity. In this review, we mainly focus on roles of basophils in immediateand delayed-onset allergic reactions. Basophils are crucial initiators, rather than effectors, in the development of IgE-mediated, chronic cutaneous allergic inflammation, which is characterized by the massive infiltration of eosinophils and neutrophils and can be elicited even in the absence of mast cells and T cells. Basophils are dispensable for the induction of IgE-mediated systemic anaphylaxis, unlike mast cells, but play a major role in IgG-mediated passive and active systemic anaphylaxis, through the release of platelet-activating factor in response to stimulation with antigen-IgG immune complexes. Thus, basophils and their products appear to be promising therapeutic targets for allergic disorders.