germinal centers
Germinal centers are important in humoral immunity, and comprise collections of activated B lymphocytes within non-thymus lymphoid tissue. Germinal centers are histologically distinct, lighter-stained centers within lymph nodules and other lymphoid tissues. [] micrograph, micrograph 2, germinal center with 'tingible body macrophages', Nodule with Germinal Center 1, Nodule with Germinal Center 2, histology, HHH atlas, diffuse and mucosa associated lymphoid tissue with germinal centers[].
Upon antigenic activation, B cells migrate to follicules where they undergo monoclonal expansion in the vicinity of follicular dendritic cells (FDC). Within three days, activated B cells mutate their antibody-encoding DNA, generating diverse clones in the germinal centre (VDJ recombination).
Follicular dendritic cells, acting as APCs, stimulate the B cells to express surface antibodies (as centrocytes) in a state of potential apoptosis, competing for FDC survival signals in a process that is considered to be dependent on the affinity of surface immunoglobulin for presented antigen (affinity maturation).
Functional B-cells receive differentiation signals from helper T cells in a process that involves isotype switching (for example, from IgM to IgG). This interaction with T cells is believed to prevent the generation of autoreactive antibodies.
Activated B cells either continue to proliferate as centroblasts (recycling hypothesis) or differentiate either into specific-antibody-producing plasma cells or into memory B cells primed to be rapidly activated by subsequent contact with the cognate antigen []diagram[].
GC morphology is specific for different stages of the proliferation/differentiation. In early stages, a network of FDCs is packed with proliferating B cells. By day 4, GCs exhibit separation into dark and the light zones, containing proliferating cells and B cells undergoing selection. After day 10, the zones dissappear. After 3 weeks, GC development typically terminates.
'Tingible body macrophages' contain phagocytosed cellular debris.
Progressive transformation of germinal centers (PTGCs) are larger than regular germinal centers and comprise mainly mantle zone lymphocytes with remnants of large germinal center cells. Mantle zones are obscured, and interfollicular areas typically contain small lymphocytes with a few immunoblasts. PTGC are usually found in the lymph nodes of patients with reactive follicular hyperplasia, though association with nodular lymphocyte predominant Hodgkin lymphoma has been described.[images]
