Wikipedia
Germinal centers or germinal centres (GC) are sites within secondary lymph nodes where mature B lymphocytes proliferate, differentiate, and mutate their antibody genes (through somatic hypermutation), and switch the class of their antibodies (for example from IgM to IgG) during a normal immune response to an infection. They develop dynamically after the activation of follicular (Fo) B cells by T-dependent antigen.
As they undergo rapid and mutative cellular division in the dark zone, B cells of the germinal center are known as centroblasts. Once these B cells have stopped proliferating, they migrate to the light zone where they are known as centrocytes, and are subjected to selection by follicular helper T (T) cells in the presence of follicular dendritic cells (FDCs). Germinal centers are an important part of the B cell humoral immune response, acting as central factories for the generation of affinity matured B cells specialized in producing improved antibodies that effectively recognize infectious agents, and for the production of durable memory B cells. Histologically, the GCs describe microscopically distinguishable parts in lymphoid tissues.