Immunology Fall 2026: Lecture 13 B cell Development and Selection

Overview

Brianne Barker traces B-cell development from RAG-mediated heavy-chain V(D)J recombination through pre-B-cell receptor signaling, light-chain rearrangement, immature B-cell formation, and central tolerance. The lecture explains allelic exclusion, surrogate light chains, receptor editing, deletion, and anergy, noting that roughly 5 million B cells exit the bone marrow daily while many more fail recombination or are removed; flow cytometry and the upcoming Attar et al. paper provide experimental context for these processes.

Key takeaways

A productive heavy-chain rearrangement ends further heavy-chain recombination because V(D)J assembly deletes intervening D segments and compatible RSSs, limiting the cell to one maternal and one paternal attempt.
The pre-B-cell receptor combines a functional heavy chain with VpreB and λ5, triggering survival, RAG shutdown, heavy-chain allelic exclusion, proliferation, and subsequent RAG reactivation for light-chain assembly.
Light-chain recombination offers up to four initial opportunities—κ1, κ2, λ1, and λ2—and receptor editing can reuse remaining V and J segments to replace a self-reactive light chain without changing the heavy chain.
Central tolerance removes or disables autoreactive immature B cells through deletion, light-chain receptor editing, or anergy before they leave the bone marrow.
Approximately 5 million B cells exit the bone marrow each day, but many more fail productive recombination or are eliminated during negative selection; some estimates place overall attrition near 95%.
Mature peripheral B cells remain dependent on T-cell help for major downstream responses, including somatic hypermutation, affinity maturation, class switching to IgG, IgA, or IgE, and strong memory-cell formation.

Chapters

0:00 Lecture Logistics and the B-Cell Development Roadmap

The flow cytometry problem set deadline moves until after the exam scheduled for the following Monday.
Students must submit questions on the Attar et al. paper by Friday; the paper connects experimental data to B-cell development mechanisms.
Brianne Barker organizes the lecture around pre-B-cell development, receptor formation, and B-cell selection.

5:00 Heavy-Chain V(D)J Recombination Creates the Pre-B-Cell Receptor

Pro-B cells initially lack surface immunoglobulin heavy and light chains but express the signaling proteins Igα and Igβ.
Heavy-chain assembly proceeds in order: D-to-J recombination followed by V-to-DJ recombination.
Both maternal and paternal chromosome 14 heavy-chain loci begin recombination, creating two opportunities to produce a functional heavy chain.
A successful heavy-chain protein is displayed with signaling components and promotes the pro-B cell into the pre-B-cell stage.

10:00 RSS Constraints Explain Why Heavy Chains Cannot Be Recombined Repeatedly

A productive heavy-chain rearrangement deletes intervening DNA, including unused D segments and their recombination signal sequences (RSSs).
After V3-D2-J1 assembly, remaining V and J segments cannot be joined because their RSSs do not satisfy the 12/23 recombination rule.
A developing B cell therefore gets only two heavy-chain attempts: one on the maternal allele and one on the paternal allele.
Failure on both heavy-chain alleles prevents a functional survival signal and causes cell death.

15:00 Surrogate Light Chains and Pre-B-Cell Signaling

The functional heavy chain pairs with the surrogate light-chain proteins VpreB and λ5 because a true light chain has not yet been made.
The heavy chain–VpreB–λ5 complex forms the pre-B-cell receptor and tests whether the heavy-chain product is a usable protein.
Pre-B-cell receptor signaling induces survival, RAG shutdown, heavy-chain allelic exclusion, and proliferation.
RAG is turned off before proliferation to prevent DNA double-strand breaks during cell division.

20:00 Allelic Exclusion Produces One Receptor Specificity per B Cell

Allelic exclusion blocks further heavy-chain rearrangement after one productive allele succeeds.
The same principle limits light-chain expression so each B cell displays a homogeneous receptor population rather than mixed specificities.
Without allelic exclusion, combinations of two heavy chains with maternal and paternal κ and λ chains could produce at least eight receptor types on one cell.
One receptor specificity per B cell allows clonal selection to classify cells as useful or self-reactive rather than producing mixed signals.

25:00 Light-Chain Rearrangement Provides Four Recombination Opportunities

After pre-B-cell proliferation, RAG is reactivated to begin V-J light-chain recombination.
Developing B cells generally attempt κ rearrangement first, using the maternal and paternal κ loci before the λ loci.
The sequence of possible attempts is κ1, κ2, λ1, and λ2, providing up to four opportunities to create a productive light chain.
A successful heavy-chain/light-chain pair forms surface IgM; failure across all four light-chain attempts causes cell death.

30:00 Immature B Cells Remain in Bone Marrow for a Final Check

A cell with a complete heavy chain and light chain becomes an immature B cell but remains in the bone marrow.
Mature B cells later coexpress IgM and IgD through alternative RNA splicing rather than additional RAG-mediated recombination.
RAG remains available during the immature B-cell stage but is permanently shut off when the cell exits the bone marrow.
The immature-to-mature transition therefore separates receptor assembly from peripheral immune activation.

35:00 Receptor Diversity Necessitates Central Tolerance

V(D)J recombination and junctional diversity generate receptors capable of recognizing an enormous range of proteins, lipids, nucleic acids, and carbohydrates.
The same breadth inevitably creates B cells whose antibodies bind endogenous self proteins.
Releasing self-reactive cells into peripheral tissues could cause autoimmune disease by directing antibodies against the body's own structures.
Brianne Barker distinguishes central tolerance in primary lymphoid organs from peripheral tolerance mechanisms that act after development.

40:00 Self-Reactivity Testing Drives Negative Selection

Immature B cells are tested against self antigens while they are still in the bone marrow.
Cells that do not respond to self antigen can leave the bone marrow, express IgM and IgD, and become mature B cells.
Self-reactive cells are retained in the bone marrow for one of three outcomes: deletion, receptor editing, or anergy.
These processes constitute negative selection and reduce the number of autoreactive B cells entering peripheral lymphoid organs.

45:00 Deletion and Light-Chain Receptor Editing Rescue Some Autoreactive Cells

Deletion eliminates strongly self-reactive B cells, particularly cells whose receptors bind self antigen with very high strength.
Receptor editing preserves the existing heavy chain but replaces the light chain to alter antigen specificity.
RAG must be reactivated for receptor editing, which is possible at light-chain loci because unused V and J segments with compatible RSSs may remain.
Editing can continue until the cell exhausts usable V-J combinations; a cell that remains self-reactive is ultimately deleted.

50:00 Anergy Silences Weakly Self-Reactive B Cells

Some weakly self-reactive B cells retain their original receptors but are rendered functionally unresponsive rather than deleted.
This state, called anergy, allows the cells to leave the bone marrow while remaining largely unable to activate.
Anergic cells may provide limited emergency breadth if an infectious microbe closely resembles self antigen.
Excess inflammatory cytokines during severe infections can reverse anergy in some cells, potentially contributing to post-infectious autoimmunity, including cases associated with severe COVID-19.

55:00 Bone-Marrow Selection Produces a Mature Peripheral B-Cell Repertoire

B-cell development combines receptor generation with negative selection before cells enter the periphery, especially the spleen and other secondary lymphoid organs.
Approximately 5 million B cells successfully leave the bone marrow each day, while some estimates suggest about 95% of developing cells fail or are eliminated.
Cells that fail heavy-chain or light-chain recombination, or remain strongly self-reactive after editing, do not join the mature repertoire.
Peripheral B cells can now encounter microbial antigen and begin clonal expansion.

1:00:00 Peripheral B-Cell Responses Require T-Cell Help

Activated B cells produce large quantities of antibody after leaving the bone marrow rather than merely displaying B-cell receptors.
Somatic hypermutation changes antibody variable regions, enabling affinity maturation and tighter antigen binding.
Class-switch recombination changes the antibody isotype from initial IgM/IgD expression to IgG, IgA, or IgE.
Affinity maturation, class switching, robust antibody production, and effective memory-cell formation generally require interaction with helper T cells; the lecture therefore transitions next to T-cell development.

Summary, takeaways, and chapters were generated by AI from the video's transcript and may contain errors. The video belongs to its creator, Brianne Barker.