Nat. Immunol. 14, 765–767 (2013)

In the version of this article originally posted online, the units in the text referring to Figure 1a were incorrect. The correct unit is “μm” and the correct text is “the receptor depicted would measure about 3 μm, compared with a resting B lymphocyte, with an average diameter of about 7 μm.” The error has been corrected in this file as of 22 August 2013.