germinal center

**Location and structure** - Germinal centers form within secondary follicles in the outer cortex of lymph nodes, the white pulp of the spleen (periarteriolar lymphoid sheath/follicles), and mucosa-associated lymphoid tissue (Peyer patches, tonsils). Primary follicles (unstimulated, naive B cells) become secondary follicles once a germinal center develops. - Dark zone: centroblasts, which are rapidly proliferating B cells. This is where somatic hypermutation (SHM) of the Ig variable region occurs, driven by activation-induced cytidine deaminase (AID). - Light zone: centrocytes, follicular dendritic cells (FDCs), and T follicular helper (Tfh) cells. Centrocytes test their mutated BCRs against antigen displayed on FDCs. B cells with higher affinity receive survival signals from Tfh cells. Low-affinity or self-reactive clones undergo apoptosis (tingible body macrophages clear the debris). - Mantle zone: a ring of surrounding naive B cells.

**Formation (T-cell–dependent response)** 1. Naive B cell binds antigen via its BCR, then internalizes and presents it on MHC II. 2. A CD4+ helper T cell (Tfh) recognizes the peptide–MHC II complex. CD40 (B cell)–CD40L (T cell) interaction plus cytokines (IL-4, IL-21, IFN-γ, etc.) activate the B cell. 3. The activated B cell migrates to the follicle and establishes the germinal center reaction.

**Key processes** - Somatic hypermutation and affinity maturation: point mutations in the V region of the Ig gene, followed by selection of high-affinity clones. - Isotype (class) switching: IgM to IgG, IgA, or IgE, directed by cytokines. It requires CD40–CD40L and AID. For example, IFN-γ leads to IgG, IL-4 to IgE/IgG4, and TGF-β/IL-5 to IgA. - Differentiation: output cells are long-lived plasma cells (which home to bone marrow) and memory B cells. - BCL-6 is the master transcription factor of the germinal center B cell and of Tfh cells. BCL-2 is normally downregulated in germinal center B cells, so selection occurs through competition for survival signals.

**Clinical correlations (high-yield)** - Follicular lymphoma: t(14;18) places BCL-2 under the IgH enhancer, causing BCL-2 overexpression and resistance to apoptosis. It is derived from germinal center B cells (CD10+, CD19+, CD20+). Nodal architecture shows a nodular, follicular pattern. - Burkitt lymphoma: t(8;14) c-MYC translocation. It is a germinal center–derived "starry sky" lymphoma (macrophages with tingible bodies). - Diffuse large B-cell lymphoma: many cases arise from germinal center B cells (GCB subtype). - Follicular (reactive) hyperplasia: enlargement of germinal centers, seen in early HIV, RA, and Sjögren syndrome, and with viral or bacterial infection. It is a benign process with preserved architecture and polarized germinal centers that contain tingible body macrophages. In contrast, follicular lymphoma has monomorphic, non-polarized follicles that lack tingible body macrophages. - Hyper-IgM syndrome: a CD40L (X-linked) or CD40 defect, or an AID defect. Germinal centers do not form (absent or poorly formed), so there is no class switching. This results in high IgM and low IgG, IgA, and IgE, with recurrent pyogenic infections and opportunistic infections such as Pneumocystis. - Bruton agammaglobulinemia (BTK defect): no mature B cells, so germinal centers are absent and lymph nodes are small with no follicles or germinal centers. - CVID: B cells are present but fail to differentiate normally, with variable germinal center formation. There is an increased risk of autoimmunity and lymphoma. - Autoimmunity: failure of germinal center selection or tolerance can allow autoreactive clones to persist (e.g., SLE, with autoantibodies showing evidence of SHM). - T-independent antigens (e.g., polysaccharide capsules, such as those of encapsulated bacteria) do not form germinal centers, so they produce mainly IgM with poor memory. This is why conjugate vaccines (polysaccharide linked to protein carrier) are used, since they recruit T-cell help and induce germinal center responses. - DiGeorge syndrome (T-cell deficiency): impaired germinal center formation due to lack of T-cell help.

**Memory pearls** - Paracortex = T cells (deficient in DiGeorge). Follicle/cortex = B cells (germinal center is here). - Medulla = plasma cells (antibody secretion) and macrophages. - Splenic follicles and germinal centers are in the white pulp. Splenic T cells are in the PALS. - Affinity maturation and class switching both occur in the germinal center and both require T-cell help (CD40–CD40L).