Antigen extraction by B cells depends on the physical properties of the antigen-coated surface. (A) On a rigid deformable substrate, antigens are recognized by the BCR and gathered into microclusters. (1) Mechanical forces pull on antigen–BCR complexes. (2 and 3) High-affinity antigens are internalized into clathrin-coated pits (2), whereas low-affinity antigens detach from the BCR upon tension and only the BCR is internalized (3): Mechanical energy is therefore used for affinity discrimination. (B) On a flexible and deformable substrate, antigens are recognized by the BCR and internalized as described in A. However, because the softer substrate allows a higher degree of deformation, affinity discrimination is less stringent. (C) On nondeformable antigens (or very stiff substrates), such as plastic or glass, B cells secrete hydrolases after polarization of lysosomes at the synapse (1), this promotes antigen release from the substrate (2) and subsequent internalization (3) of soluble antigens.