Molecular dissection of an antigen-specific immune response

Selection defines the repertoire of the primary response, and also which cells differentiate and enter the pool of the memory cells. Whereas the primary response is drawn from the germline repertoire, the memory response consists overwhelmingly of cells which have been hypermutated and selected to express receptors of high affinity. The accumulation of somatic mutations over time indicates that the activation of the memory cell by antigen also reactivates the hypermutation mechanism. Since the frequency of somatic mutations correlates with an increase in antibody affinity, an efficient selection operates not only on the primary B cells when first stimulated by antigen, but also on the memory cells after secondary or tertiary immunization.