**1. Introduction**

Activation of mast cells and basophils via high-affinity IgE receptors (FcεRI) on the cell surface plays an essential role in allergic reactions. Multivalent allergens induce the aggregation or cross-linking of IgE-bound FcεRI to trigger their activation [1]. Activated mast cells and basophils release preformed chemicals (e.g., histamine, serotonin) and protein inflammatory mediators (e.g., proteases, tumor necrosis factor (TNF)), and de novo synthesize and secrete arachidonic acid-derived lipids, cytokines, chemokines, and growth factors [2,3]. These factors promote type 2 inflammation in allergic individuals. In this review, we will discuss histamine-releasing factor (HRF)-mediated regulation of mast cell/basophil activation via FcεRI and its roles in allergic and other immune diseases.
