The human basophil: a new appreciation of its role in immune responses.

Basophilic granulocytes (usually referred to as basophils) are a small population of peripheral blood leukocytes containing cytoplasmatic granules that stain with basophilic dyes. They were first described in 1879 by Paul Ehrlich,1 who 1 year earlier had found a morphologically similar cell type present in tissues that he termed Mastzellen.2 Based on their similarity to mast cells, basophils have often been considered (and neglected) as minor and possibly redundant “circulating mast cells.” Like mast cells, basophils possess high-affinity immunoglobulin (Ig) E receptors (FceRI) that are cross-linked upon engagement of receptor-bound IgE with corresponding antigens (“allergens”), resulting in the release of a number of mediators that are in part common for both cell types. Until recently, it has been very difficult for most laboratories to obtain basophils without major contaminating cell populations, because the percentage of basophils in peripheral blood is low (, 1%) and they share physicochemical properties with other blood cells. This lack of satisfactory purification protocols has considerably hampered basophil research and negatively affected the interest in this cell type. Nevertheless, recent findings have provided new insights into the possible role of basophils in allergic disease and immunity to pathogens. Most notably, the discovery that basophils rapidly produce large amounts of the regulatory cytokines interleukin (IL)-4 3,4 and IL-13,5-7 together with the constitutive expression of CD40L 8,9 and CCR310 on their surface, has fueled speculations that extend beyond their recognized role as effector cells in IgE-mediated reactions. As discussed in this review, the increasing knowledge about the chemokine network has provided a robust framework for understanding the mechanisms underlying basophil recruitment to the tissues in allergic diseases. This development has been instrumental in identifying very promising new targets for the treatment of asthma and other allergic diseases. We also review recent aspects regarding basophil growth and development and their role in allergic late-phase reactions (LPRs). Furthermore, we have included a section on basophil signal transduction, which expands our understanding of the mechanisms operating downstream of the high-affinity IgE receptor and are also expected to yield new and better targets for pharmacologic intervention. Finally, we discuss the possible roles of basophils in innate immunity and finish with a section on the currently available tools for basophil research.