germinalcentrum

A germinal center is a transient microanatomical structure within secondary lymphoid organs where mature B cells proliferate, differentiate, and mutate their antibody genes during an immune response. Germinal centers form in response to T cell–dependent antigens and are located in lymph nodes, the spleen, and mucosa-associated lymphoid tissue. They rely on interactions with follicular dendritic cells that present antigen and with T follicular helper cells that provide help through CD40–CD40L signaling and cytokines such as IL-21.

The germinal center has two distinct zones. The dark zone contains rapidly proliferating B cells known as centroblasts that undergo somatic hypermutation of immunoglobulin genes, a process driven by activation-induced cytidine deaminase (AID). The light zone contains non-dividing centrocytes that compete for antigen displayed on follicular dendritic cells and receive survival signals from T follicular helper cells. In this phase, B cells with higher-affinity B cell receptors are selected for, while lower-affinity clones are deleted.

Class-switch recombination and ongoing affinity maturation occur during the germinal center reaction, shaping the antibody response. Successful B cells emerge as high-affinity memory B cells or long-lived plasma cells that migrate to the bone marrow or other sites to provide lasting humoral immunity. Germinal center reactions are tightly regulated by transcription factors such as BCL6 and by interactions between B cells, FDCs, and Tfh cells; dysregulation can be associated with autoimmune conditions or B-cell lymphomas.