A range of Cε3-Cε4 interdomain angles in IgE Fc accommodate binding to its receptor CD23

The antibody IgE plays a central role in allergic disease, functioning principally through two cell-surface receptors: FcεRI and CD23. FcεRI on mast cells and basophils mediates the immediate hypersensitivity response, whilst the interaction of IgE with CD23 on B cells regulates IgE production. Crystal structures of the lectin-like 'head' domain of CD23 alone and bound to a subfragment of IgE consisting of the dimer of Cε3 and Cε4 domains (Fcε3-4) have recently been determined, revealing flexibility in the IgE-binding site of CD23. Here, a new crystal form of the CD23-Fcε3-4 complex with different molecular-packing constraints is reported, which together with the earlier results demonstrates that conformational variability at the interface extends additionally to the IgE Fc and the quaternary structure of its domains. © 2014 International Union of Crystallography.