The germinal center reaction. During immune responses, antigen-responsive B cells proliferate within lymphoid follicles, leading to formation of germinal centers. Rapidly cycling cells are located in the dark zone, where somatic hypermutation of Ig V regions occurs. Cells then exit the cell cycle and migrate to the light zone, where cell fates are determined by interaction with antigen-bearing FDC and antigen-specific T cells or undergo cell death. B cells with high-affinity receptors undergo class switch recombination and may either exit the germinal center to differentiate into memory cells and plasma cells or re-enter the dark zone for additional rounds of replication and hypermutation. Some B cells may also undergo V(D)J recombination.