AFFINITY SELECTION AND REPERTOIRE SHIFT - PARADOXES AS A CONSEQUENCE OF SOMATIC MUTATIONS

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 cellula r that involves the clonal expansion of B cells expressing a surface i mmunoglobulin with a higher affinity for antigen compared to their com petitors. In this review we focus on the conditions for affinity selec tion during the establishment, expansion and memory phases of the immu ne response. We postulate that somatic mutation evolved prior to affin ity selection and we present a model for selection of B cells in germi nal centres. We also discuss the possibility that antibody repertoire shift occurs during the memory maintenance phase. Finally we argue tha t a significant affinity selection and a selection for polyclonality o f immune responses occur during this stage of the immune response.