B Cells Use Mechanical Energy to Discriminate Antigen Affinities

High-affinity, protective antibodies made by B cells are critical for providing long-term protection against reinfection. In order to produce antibodies, B cells must first bind to and extract antigens from the surface of antigen-presenting cells. Using an in vitro system that allows B cells to bind to antigenladen, flexible membranes, Natkanski et al. (p. [1587][1], published online 16 May) show that antigen extraction relies on myosin IIA–mediated contractile forces that pull upon the antigen-presenting membrane. These forces break the antigen-receptor bonds if affinity is low, thus ensuring that B cells only extract, internalize, and presumably respond to, high-affinity antigens. [1]: /lookup/doi/10.1126/science.1237572