Multiple players in mouse B cell memory. 
B cell memory has long been considered the attribute of the sole IgG-positive B cell subset.  Since a few years, and due to new B-cell subset identification procedures, increasing heterogeneity has been identified among the memory B cell pool.  IgM-positive cells and germinal center-independent subsets are recent additions to the field.  This review describes the diversity of memory B cells, as well as controversial issues on their relative contribution to the recall response.  The impact of a protracted germinal center response to the specific mobilization of IgM memory B cells is proposed.