B cell diversification and differentiation in the periphery.

T and B lymphocytes make physical contact (6). Though both swift and brief (4, 7), this encounter initiates a complex program of B cell differentiation leading to specialization for early antibody production or immune memory. In the spleen, B cells initially proliferate in the T cell-rich periarteriolar lymphoid sheath (PALS) and then either develop locally into foci of antibody-secreting cells or migrate to the nearby lymphoid follicle to initiate germinal center (GC) formation (3-5).