FIGURE 1 | Generation of memory cells and effector cells.

a | Development of memory B cells and effector B cells (plasma cells) occurs in two phases. Short-lived plasma cells that make mostly IgM (but some IgG) are generated during the primary response and occupy sites, such as the splenic red pulp or lymph node medulla. B cells are also seeded to follicles to form germinal centres in this early phase. The second phase involves the formation of the memory B-cell pool and seeding of long-lived plasma cells to the bone marrow (making predominantly switched isotype antibodies). Plasma cells are terminally differentiated and do not give rise to memory cells. All arrows are driven by antigen and T-cell help. b | Development of memory T cells (CD4 and CD8). After activation, cells differentiate into effector T cells. Memory T cells might be generated by divergence from this pathway or directly from effector T cells^{17, 50, 51, 52}. There might be two subsets of memory cells⁵³: quiescent, central memory cells that recirculate from blood to secondary lymphoid organs, and effector memory cells that migrate through tissues^{54, 55} and deliver a very rapid response on reactivation with antigen.

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