B cell activation involves nanoscale receptor reorganizations and inside-out signaling by Syk.

Binding of antigen to the B cell antigen receptor (BCR) initiates a multitude of events resulting in B cell activation. How the BCR becomes signaling-competent upon antigen binding is still a matter of controversy. Using a high-resolution proximity ligation assay (PLA) to monitor the conformation of the BCR and its interactions with co-receptors at a 10-20 nm resolution, we provide direct evidence for the opening of BCR dimers during B cell activation.