Figure 2.

The germinal center reaction. After antigenic exposure, B and T cells are activated in their respective zones within peripheral lymphoid tissue. Activated cells then migrate to the B/T boundary where they interact with one another. T cells impart further activation signals onto cognate B cells, leading to the stimulation of pauciclonal GC founding populations. On GC seeding, founder cells are termed centroblasts and are characterized by the down-regulation of the immunoglobulin receptor from their cell surface and intense levels of proliferation and AID-mediated SHM. These cells then migrate to the light zone of the GC, where they reexpress the immunoglobulin receptor and are termed centrocytes. The light zone provides a competitive environment whereby centrocytes are positively selected based on their ability to bind antigen expressed on the surface of follicular dendritic cells (FDCs) in the form of immune complexes. FDCs and CD4⁺ T cells provide positively selected clones with the signals needed to exit the GC as a plasma or memory cell. Negatively selected cells die via apoptosis, where they are phagocytosed by local macrophages (not shown). A portion of cells may re-enter the dark zone to undergo further rounds of proliferation and SHM.