CD23 bridges the gap

The structure of the low-affinity human immunoglobulin E (IgE) receptor CD23, revealed on page 751, accounts for its provocative behavior. Hibbert and colleagues show that soluble CD23 (sCD23) can simultaneously bind IgE and the complement receptor CD21 on B cells. This binding dexterity helps explain how CD23 and IgE cooperate to ramp up the synthesis of more IgE.

The production of IgE—the signature antibody of allergic diseases—is enhanced when sCD23 (which is cleaved from the surface of activated B cells by endogenous or pathogen-derived proteases) binds to CD21 on other B cells. But it was not clear why sCD23 binding to CD21 enhanced the production of IgE but not other antibody isotypes, as CD21 is present on all B cells.The group used NMR spectroscopy to show that sCD23 can...