Basophils are the least common granulocytes which possess basophilic granules in their cytoplasm. Although they were first documented by Paul Ehrlich more than 140 years ago, their functional significance had long been an enigma, partly because of their rarity and the lack of analytical tools for basophils. In addition, basophils had been erroneously regarded as "blood-circulating mast cells," due to their similarities to tissue-resident mast cells, such as the expression of high-affinity IgE receptor (FcεRI) on their cell surface and the release of histamine upon their activation. Nevertheless, basophils and mast cells differ in a number of aspects. Basophils circulate in the peripheral blood under homeostatic conditions while mast cells typically reside in peripheral tissues and are hardly detected in the peripheral blood. Transcriptomic analyses revealed that basophils display distinct transcriptomic profiles from mast cells in both mice and humans. [1][2][3] Recent development of analytical tools for basophils has enabled us to identify nonredundant functions of basophils in multiple immune reactions, including allergic inflammation, and protective immunity against parasites. [4][5][6][7][8] In addition, the involvement of basophils is also demonstrated in several nonallergic disorders, such as lupus nephritis, 9-11 chronic obstructive pulmonary disease, 12 allograft rejection, 13,14 and carcinogenesis. 15,16 In this review, we summarize recent advances in our understanding of basophil biology, especially focusing on the roles of basophils in the context of allergic inflammation (Boxes 1 and 2).
| ONTOG ENY OF BA SOPHIL SAlthough it is widely accepted that basophils differentiate from hematopoietic stem cells (HSCs), 17,18 the pathway of their differentiation