Affinity selection and repertoire shift: paradoxes as a consequence of somatic mutation?

Affinity selection of antibodies during immune responses relies on two mechanisms, one molecular that involves the targeted introduction of somatic mutations into rearranged immunoglobulin genes and one cellular that involves the clonal expansion of B cells expressing a surface immunoglobulin with a higher affinity for antigen compared to their competitors. In this review we focus on the conditions for affinity selection during the establishment, expansion and memory phases of the immune response. We postulate that somatic mutation evolved prior to affinity selection and we present a model for selection of B cells in germinal centres. We also discuss the possibility that antibody repertoire shift occurs during the memory maintenance phase. Finally, we argue that a significant affinity selection and a selection for polyclonality of immune responses occur during this stage of the immune response.