A range of C∊3–C∊4 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: Fc∊RI and CD23. Fc∊RI 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 C∊3 and C∊4 domains (Fc∊3-4) have recently been determined, revealing flexibility in the IgE-binding site of CD23. Here, a new crystal form of the CD23–Fc∊3-4 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.