Conformational plasticity at the IgE-binding site of the B-cell receptor CD23

•The IgE antibody plays a central role in allergic disease.•Binding of IgE to the B-cell receptor CD23 regulates IgE synthesis.•Seven crystal forms of the IgE-binding head domain of CD23 have been analyzed and compared.•The thirty-five independent structures reveal conformational plasticity in two IgE-binding loops. IgE antibodies play a central role in allergic disease. They recognize allergens via their Fab regions, whilst their effector functions are controlled through interactions of the Fc region with two principal cell surface receptors, FcɛRI and CD23. Crosslinking of FcɛRI-bound IgE on mast cells and basophils by allergen initiates an immediate inflammatory response, while the interaction of IgE with CD23 on B-cells regulates IgE production. We have determined the structures of the C-type lectin “head” domain of CD23 from seven crystal forms. The thirty-five independent structures reveal extensive conformational plasticity in two loops that are critical for IgE binding.