Basophils are potent antigen-presenting cells that selectively induce Th2 cells.

Basophils and mast cells are important effector cells in helminth-infected host and IgE-mediated allergic inflammation. Although they have the same progenitors, basophils and mast cells complete their terminal differentiation in the bone marrow and peripheral tissues, respectively, and only basophils circulate in the blood. Although it is recognized that basophils are important for Th2 responses, and it is also well established that IL-4 is required for Th2 differentiation from naïve CD4(+) T cells, the nature of the cells that produce "early" IL-4, remained elusive until recently. Three groups independently demonstrated that basophils are the predominant APC in inducing Th2 response against helminth parasites and allergens. Basophils express MHC class II and CD80/86, have the potential to take-up and process protein Ag (particularly Ag-IgE complex) and to present peptide in the context of MHC class II, and to produce IL-4. These Ag-pulsed basophils induce the development of Th2 cells both in vitro and in vivo. Thus, basophils contribute to Th2/IgE response by the production of IL-4 and presentation of MHC class II/peptide complex to naïve CD4(+) T cells, in contrast to the Th1-inducing action of DC. In this review, we summarize what is known regarding basophil function in allergy and parasite infection, examine the novel Ag-presenting function of basophils and discuss potential clinical implications of this finding.