CD23, the low affinity IgE receptor, is a type II transmembrane protein expressed on hematopoietic cells and contains two important extracellular domains, the IgE binding lectin head, and the coil-coiled stalk, responsible for CD23's trimeric structure. Mouse models have shown that trimetric membrane bound CD23 regulates IgE synthesis, which ability is lost when CD23 is shed. Recently it was shown that the CD23 sheddase is ADAM10, and shedding is controlled by the trimer stability. In this study, we examined the interactions of CD23 and ADAM10. We found that the binding of ADAM10, using either cell lysates containing ADAM10 or rADAM10 (Janes et al. 2005. Cell 123:291-304.), to CD23 is unlike any other protein that interacts with CD23, in that it binds the stalk region, between amino acids 139-157 in a protease independent manner. When CD23 is destabilized, with the anti-stalk antibody, 19G5, ADAM10 binding increased. Stabilizing CD23 with IgE had the opposite affect. These studies will be further examined with surface plasmon resonance.