B-Cell Development, Germinal Centers & Antibody Diversity

Marrow development and its checkpoints

Overview. B cells develop in the bone marrow through four stages, each ending in a checkpoint that tests the receptor chain just built. Mature naive B cells then leave the marrow and home to the follicles of lymph nodes, spleen and MALT to wait for antigen.

Pro-B cell. The heavy-chain locus (chromosome 14) rearranges first. RAG1 and RAG2 cut DNA at recombination signal sequences; a D segment joins a J segment, then a V segment joins the DJ unit. The breaks are sealed by nonhomologous end joining (Artemis, DNA-PK). Null RAG mutations abolish both B and T cells, giving T−B−NK+ SCID; hypomorphic mutations cause Omenn syndrome, with erythroderma, eosinophilia and high IgE.

Pre-B cell. A productive μ heavy chain pairs with a surrogate light chain (VpreB plus λ5) and the signaling chains Igα/Igβ (CD79a/CD79b) to form the pre-BCR. Its signal travels through Bruton tyrosine kinase (BTK) and drives proliferation, light-chain rearrangement and allelic exclusion, so each B cell expresses one heavy-chain allele and a single specificity. BTK mutation (X-linked agammaglobulinemia) halts development here, leaving few circulating CD19+/CD20+ B cells and low levels of every immunoglobulin class.

Immature B cell. The light chain rearranges V to J (there is no D segment), kappa first and lambda only if kappa fails (kappa on chromosome 2, lambda on 22). The completed IgM molecule appears on the surface. Central tolerance acts now: a clone binding self-antigen strongly re-expresses RAG and tries receptor editing with a fresh light chain; if it remains self-reactive it undergoes clonal deletion by apoptosis. Weak recognition of soluble self-antigen tends to cause anergy instead.

Mature naive B cell. A single VDJ-Cμ-Cδ primary transcript is alternatively spliced, so the cell displays IgM and IgD with identical specificity. No DNA is lost, unlike later class switching.

Generating antibody diversity

• Combinatorial joining: each B cell picks one V, D and J segment for its heavy chain and one V and J for its light chain.
• Junctional diversity: joining is deliberately imprecise. Exonuclease trimming, palindromic P-nucleotides and nontemplated N-nucleotides added by terminal deoxynucleotidyl transferase (TdT) alter the junction, which encodes CDR3, the most variable antigen-binding loop.
• Heavy-light pairing: random combination of any heavy chain with any light chain multiplies the repertoire again.
• Somatic hypermutation: after antigen exposure, point mutations in germinal centers fine-tune the V regions.

The first three happen in the marrow before antigen is ever met; hypermutation is antigen-driven. Class switching changes effector function, not specificity.

TdT is made only by early lymphoid precursors, so nuclear TdT marks the blasts of acute lymphoblastic leukemia and lymphoblastic lymphoma (B-ALL blasts are typically also CD10+ and CD19+).

T-dependent activation and the germinal center

Tfh help. A B cell binds protein antigen through its BCR (with the CD19/CD21 coreceptor), internalizes it and presents its peptides on MHC II. A CD4 T follicular helper cell (CXCR5+, Bcl-6-driven) recognizing that peptide replies with CD40L binding CD40 plus IL-21 and IL-4. The activated B cell seeds a germinal center in the follicle.

Follicular dendritic cells. FDCs are stromal cells, not marrow-derived APCs. They hold immune complexes on complement and Fc receptors so B cells can test intact antigen; they do not present processed peptide on MHC II. Dead losers are cleared by tingible-body macrophages.

Affinity maturation and outputs. Repeated mutation-selection cycles raise antibody affinity as the response matures. Long-lived plasma cells home to the marrow and secrete antibody for years, keeping titers up after infection or vaccination; they lose surface CD20 and express CD38 and CD138. Memory B cells launch faster, larger, higher-affinity IgG responses on re-exposure, which is why a secondary response outperforms the IgM-dominated primary response.

Class switching and AID

Activation-induced cytidine deaminase (AID). AID converts cytosine to uracil in DNA. In V regions this seeds point mutations (somatic hypermutation). In switch regions upstream of constant genes it triggers class switch recombination, which deletes the intervening constant genes so the same VDJ now joins Cγ, Cα or Cε. Because DNA is removed, switching is irreversible for that cell.

Clinical correlations

Hyper-IgM syndromes. Loss of either partner of the switch machinery leaves normal or high IgM with low IgG, IgA and IgE and recurrent sinopulmonary bacterial infections.

T-independent antigens. Repetitive capsular polysaccharides cross-link many BCRs, largely on splenic marginal-zone B cells, and trigger mostly IgM with little switching or memory; children under 2 respond poorly. Coupling the polysaccharide to a protein carrier lets B cells present carrier peptides to Tfh cells, giving switched IgG and memory. This is the basis of the Haemophilus influenzae type b, pneumococcal (PCV) and meningococcal conjugate vaccines.

Germinal-center B cells are the origin of many lymphomas: AID-induced DNA breaks favor translocations into the IgH locus on chromosome 14, such as t(8;14) in Burkitt lymphoma. The t(14;18) of follicular lymphoma arises earlier, from a RAG error during V(D)J joining in marrow B-cell precursors.