Affinity maturation of B cell receptors (BCRs) is an important component of germinal centre (GC) responses. Direct competition between GC B cells for antigen is thought to select for high-affinity clones. However, this does not explain how BCR affinities can increase over the course of an infection, when large amounts of antigen are produced for prolonged periods of time. This study proposes a new model for affinity maturation, whereby GC B cells compete with their own secreted antibodies for access to antigen. Experiments with monoclonal antibodies of defined affinities provided support for the authors' model, which explains how a directional selection pressure can be maintained in the GC. Notably, this model indicates that communication between distant GCs might be facilitated by antibodies, rather than by B cell migration, and can also explain how the GC response is terminated: eventually, the 'masking' of antigen by high affinity antibodies is too strong to allow for B cell survival.