The generation of high-affinity antibodies depends on the ability of B cells to acquire antigens from antigen-presenting cells. In Science, Tolar and colleagues show that B cells acquire antigens by dynamic contractions that pull out and engulf the presenting membranes. By comparing plasma membrane sheets and planar lipid bilayers, the authors show that flexibility of the presenting membrane is important. B cells use actomyosin contractility to pull out and invaginate the presenting membranes and internalize the antigen through clathrin- and actin-dependent processes. Diminishing the strength of myosin IIa contraction improves the internalization of low-affinity antigens. This suggests that although bonds between the B cell receptor (BCR) and antigen in small microclusters might break under the pulling forces, larger microclusters resist the contractile forces for long enough (20–30 seconds) to allow the association of clathrin and internalization. Thus, contractile forces mechanically test the strength of BCR binding and provide a mechanism for affinity discrimination.

Science (16 May 2013) doi:10.1126/science.1237572