Bcell germinal centre pathway
The germinal centre is where antibody quality is made. Antigen bound by the IgM/IgD B-cell receptor is transduced through Igα/Igβ (CD79a/CD79b) ITAMs by Lyn and Syk, amplified by the CD19–CD21–CD81 complement co-receptor when C3d is deposited, and damped by CD22, FcγRIIB, SHP-1, SHIP-1 and PTEN. Btk, BLNK and PLCγ2 generate IP₃/Ca²⁺ and DAG, driving calcineurin–NFAT, PKCβ–CARMA1–BCL10–MALT1–IKK–NF-κB and ERK/JNK–AP-1, while PI3Kδ–AKT sets the survival threshold with Bcl-2, Mcl-1 and c-Myc. Survival signals arrive independently from BAFF and APRIL via BAFF-R, TACI and BCMA. At the follicle border, Tfh cells deliver CD40L, ICOS and IL-21, and the CXCR5/CXCL13 and CXCR4/CXCL12 axes with S1PR2 partition the germinal centre into a dark zone (proliferation and AID-driven somatic hypermutation, with UNG, APE1, MSH2/MSH6 and Pol η) and a light zone (affinity-based selection on follicular dendritic cells). Bcl-6, Pax5 and Bach2 hold the germinal-centre programme; IRF4, Blimp-1 and XBP1 break it to make plasma cells. The output is class-switched IgG, IgA and IgE, memory B cells, and long-lived plasma cells in the marrow niche. Click any protein for the matching Assay Genie In Vivo antibody or ELISA kit.
The BCR signalosome and its co-receptors. Membrane IgM and IgD have no signalling tail of their own; they pair with the Igα/Igβ heterodimer (CD79a/CD79b), whose ITAMs are phosphorylated by Lyn and recruit Syk. Syk phosphorylates the adaptor BLNK/SLP-65, which nucleates Btk and PLCγ2. PLCγ2 cleaves PIP₂ to IP₃ and DAG: IP₃ releases store Ca²⁺ and activates calcineurin–NFAT, while DAG activates PKCβ, which assembles the CARMA1–BCL10–MALT1 complex on IKK to release NF-κB. In parallel PI3Kδ makes PIP₃ to recruit AKT, and ERK and JNK drive AP-1. Signal strength is set by three brakes: CD22 and FcγRIIB recruit SHP-1 and SHIP-1 to ITIMs, and PTEN removes PIP₃. Conversely, complement-tagged antigen bearing C3d engages the CD21 (CR2)–CD19–CD81 complex and lowers the activation threshold by orders of magnitude — the molecular reason adjuvants that fix complement are so effective.
Survival, T-cell help and follicle entry. Independently of antigen, BAFF/BLyS and APRIL signal through BAFF-R, TACI and BCMA to sustain Bcl-2 and Mcl-1 and to set the size of the peripheral B-cell pool. Activated B cells upregulate CCR7-independent CXCR5 and move to the T–B border, where Tfh cells provide CD40L to CD40, ICOS to ICOSL, and IL-21, IL-4 and IL-6 to their receptors. The B cell reciprocates by presenting peptide on MHC class II with CD86 co-stimulation — a genuinely bidirectional synapse. TLR7 and TLR9 supply a third, innate signal for nucleic-acid-containing antigen, and PD-1 on Tfh cells restrains the amount of help delivered.
The germinal centre: dark zone and light zone. Bcl-6 represses Blimp-1, the DNA damage response and cell-cycle checkpoints, licensing the germinal-centre state; Pax5 and Bach2 maintain B-cell identity. S1PR2 confines the reaction to the follicle, while CXCR4/CXCL12 holds centroblasts in the dark zone and CXCR5/CXCL13 pulls centrocytes into the light zone. In the dark zone, AID (AICDA) deaminates cytosine to uracil in the variable region; UNG excises it and APE1 nicks the abasic site, while MSH2/MSH6 and the error-prone polymerase Pol η convert the lesion into point mutations — somatic hypermutation. In the light zone, the mutated BCR competes for antigen displayed on follicular dendritic cells; the highest-affinity clones capture more antigen, present more peptide, win more CD40L and IL-21 help, re-express c-Myc and cycle back to the dark zone. Losers die by Fas-dependent and neglect-driven apoptosis.
Class switching, output and the In Vivo opportunity. The same AID activity at the switch regions of the IgH locus creates staggered double-strand breaks that are joined by non-homologous end joining, producing IgG, IgA or IgE under cytokine instruction (IL-4 for IgG1/IgE, TGF-β/APRIL for IgA). Cells that break the Bcl-6 programme upregulate IRF4, Blimp-1 and XBP1 and become plasmablasts and CD138+ long-lived plasma cells that home to a marrow niche of CXCL12, APRIL and IL-6; others become CD27+ memory B cells. Because so much of this axis is surface- and cytokine-based, it is unusually tractable in vivo: anti-CD19, anti-CD20, anti-CD45R/B220 (RA3-6B2), anti-CD22 (Cy34.1), anti-CD24, anti-CD38 (NIMR5), anti-CD16/CD32 (2.4G2), anti-CD40, anti-CD86, anti-MHC-II (M5/114), anti-ICOSL (CD275), anti-IL-4 (11B11), anti-IL-6 (MP5-20F3), anti-IL-21R (4A9), anti-CXCR4 (12G5), anti-PD-1 (29F.1A12), anti-CD4 (GK1.5) and IgM isotype controls all perturb it, while ELISA kits quantify every kinase, transcription factor, repair enzyme and secreted isotype. For research use only; not for diagnostic or therapeutic procedures.
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