B-cell memory: are subsets necessary?

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Abstract
In this Opinion article, a new model for the generation and the maintenance of memory B cells is proposed. The model involves these cells being continuously produced by the germinal centre throughout an immune response, with B cells that are produced later in the response being fitter and therefore having a survival advantage. B-cell memory is provided by populations of quiescent memory B cells and long-lived plasma cells. Whereas it is clear that both of these cell populations arise from germinal centres, the signals and circumstances that trigger germinal-centre B cells to enter and then persist in memory compartments are poorly defined. Here, I propose that germinal centres produce memory B cells and plasma cells throughout the immune response and that memory B cells arise by the emigration of B cells that are chosen at random from the pool available in the germinal centre. The ability of such emigrants to survive as memory B cells depends on their germinal-centre 'history', with the persistence of high-affinity B-cell variants being favoured.