After activation by antigen, mature naive B cells migrate to the edge of the follicles, where they receive help from cognate T cells. If the B cells express the appropriate molecules, such as a combination of B-cell lymphoma 6 (BCL-6), inducible T-cell co-stimulator ligand (ICOSL), CD40 and B-lymphocyte-induced maturation protein 1 (BLIMP1), the interaction of the B cells and T cells leads to the formation of short-lived plasma cells and to the establishment of germinal centres in the follicles. In the germinal centre, proliferating antigen-specific B cells (known as centroblasts) are localized at one pole (the dark zone), whereas their non-proliferating immunoglobulin-expressing counterparts (known as centrocytes) localize at the other pole (the light zone). Centrocytes and centroblasts cycle within the germinal centre in a chemokine-driven process. Centrocytes can differentiate into memory B cells or plasma cells, or undergo apoptosis if they fail to receive an antigen-mediated survival signal. Although expression of BLIMP1 is crucial for the formation of plasma cells, the factors that control memory B-cell formation are less well defined. Memory B cells recirculate in the periphery, whereas germinal-centre-derived plasma cells accumulate preferentially in the bone marrow.