Immunoglobulin E (IgE) binds with high affinity to its receptor, FcεRI, on mast cells and basophils, and cross-linking of allergen-specific IgE by minute amounts of multivalent allergen stimulates a powerful and immediate allergic reaction. In this issue of
Science Signaling
, Zhang
et al.
report the three-dimensional structures of the human and murine receptors, with and without bound IgE-Fc, to reveal some intriguing differences between mouse and human in this critical antibody-receptor interaction.