Immunology Fall 2026: Lecture 12 B cell Development

Overview

Brianne Barker explains how conventional B2 cells develop in bone marrow by coordinating irreversible immunoglobulin gene rearrangements with survival signals and quality checks. The lecture follows heavy-chain recombination in pro-B cells, allelic exclusion across two chromosome 14 copies, and formation of the pre-B-cell receptor using VpreB and λ5; a successful heavy chain triggers survival and proliferation before light-chain rearrangement resumes.

Key takeaways

RAG1 and RAG2 are expressed only at specific stages of developing B and T cells, while recombination enzymes work together in a DNA-holding complex to reduce dangerous off-target joins.
Bone-marrow growth factors, especially IL-7, keep developing lymphocytes alive before they have a B-cell receptor capable of signaling.
B-cell receptors bind antigen through membrane immunoglobulin but rely on Igα/CD79a and Igβ/CD79b for intracellular signaling and tyrosine-based activation.
A B cell rearranges D-to-J on both heavy-chain alleles, but a functional V-to-DJ rearrangement on one allele triggers RAG shutdown and chromatin closure, enforcing allelic exclusion.
A frameshift can disable the first heavy-chain rearrangement; the second chromosome provides another attempt, but failure of both alleles leads to cell death.
The pre-B-cell receptor uses a functional heavy chain paired with VpreB and λ5; its signal validates the heavy chain, promotes survival and proliferation, and keeps RAG off during DNA replication.

Chapters

0:00 Course Schedule, Exam Scope, and Upcoming B-Cell Topics

The V(D)J problem set is due on Moodle by 5:00 p.m.; the updated schedule moves immunology techniques, including flow cytometry, until after exam one.
The week covers B-cell development, B-cell selection, and a primary-literature discussion; students should read the posted paper and attempt its questions before Friday.
Exam one is scheduled for the following Monday, and Brianne Barker plans to poll students about review-session times.

5:00 V(D)J Recombination, Irreversible DNA Changes, and RAG Control

B cells assemble variable regions from V, D, and J segments in heavy chains and V and J segments in either κ or λ light chains, using RAG1 and RAG2.
Junctional diversity can add P and N nucleotides; DNA rearrangements are irreversible, unlike RNA splicing that enables IgM and IgD or membrane and secreted forms.
RAG1, RAG2, Artemis, and TdT work together in a complex that holds DNA ends, reducing the chance of inappropriate DNA joining.
RAG expression is restricted to developing B and T cells and switched on only during specific developmental stages, limiting DNA-break risks and tumor-causing errors.

11:34 B-Cell Development Takes Place Before Antigen Exposure

B-cell development builds on V(D)J recombination by placing the process in a cell and coordinating when each rearrangement occurs.
The events discussed occur in the primary lymphoid organ, bone marrow, before antigen-driven activation in peripheral tissues.
The lecture focuses on conventional B2 cells; B1 cells are a distinct B-cell type with some developmental differences.

14:12 Hematopoietic Stem Cells Supply the B-Cell Lineage

Hematopoiesis begins during gestation and continues throughout life, with hematopoietic stem cells able to generate blood-cell lineages including lymphoid precursors and B cells.
Bone marrow becomes a major hematopoietic site from roughly five months of gestation and remains active in adults.
Bone marrow transplantation can restore hematopoiesis after an immune system is destroyed or severely damaged, such as by radiation or cancer treatment.

17:30 Stem-Cell Transplants Can Rebuild an Entire Immune System

In a mouse experiment, 20 transplanted stem cells restored the immune system in all reported mice, while 10 cells succeeded in 90%.
Even one stem cell restored the immune system in about 21–22% of mice, illustrating the regenerative capacity of hematopoietic stem cells.
Brianne Barker notes that injection difficulty may cause underestimates and that purified stem-cell transplants are now more common than whole bone-marrow transplants.

21:58 Bone Marrow Provides Survival Signals to Developing B Cells

B-cell development proceeds in bone marrow, where developing cells move from having no B-cell receptor to expressing receptor components.
About five million B cells leave the bone marrow each day.
Early lymphocytes lack a B-cell receptor, so they depend on other growth-factor signals supplied by bone-marrow cells.

26:16 IL-7 Supports the Survival and Commitment of Lymphocytes

Developing B cells are highly dependent on continual survival signals; without them, they die.
Bone-marrow cells provide membrane-associated signals and secreted cytokines, including IL-7.
IL-7 signaling supports lymphocyte survival and proliferation and contributes to commitment to the B-cell lineage.

30:43 B-Cell Receptor Signaling Requires More Than Antigen Binding

A mature B cell is generally activated when antigen cross-links its B-cell receptors, producing signals that can lead to antibody production.
The B-cell receptor is an antibody with a transmembrane domain, but its membrane-bound immunoglobulin lacks an intracellular signaling domain.
The lecture uses receptor tyrosine kinases (RTKs) to explain induced proximity: ligand binding brings receptor components together so kinase activity can phosphorylate tyrosines.

34:06 RTK Signaling: Ligand Binding Brings Kinases and Tyrosines Together

An RTK contains a ligand-binding receptor component, tyrosine residues that can be phosphorylated, and kinase activity that adds phosphate groups.
Separate transmembrane receptor chains diffuse through the membrane; ligand binding brings them together and enables phosphorylation.
The induced-proximity model provides a framework for understanding how receptor engagement initiates intracellular signaling.

38:23 Igα and Igβ Supply the B-Cell Receptor’s Signaling Machinery

B-cell receptor signaling depends on the partner proteins Igα and Igβ, also called CD79a and CD79b, which carry intracellular signaling motifs.
Charged residues in the proteins’ transmembrane domains help them associate with membrane immunoglobulin.
Antigen cross-linking brings B-cell receptors and their associated signaling machinery together; other proteins, including complement receptors, can strengthen activation.
Downstream signaling changes gene transcription, cell function, differentiation, and activation.

45:45 Pro-B Cells Begin Heavy-Chain D-to-J and V-to-DJ Rearrangement

The lecture begins the staged developmental sequence with pro-B cells, which have committed to the B-cell lineage but do not yet display a B-cell receptor.
Pro-B cells first rearrange heavy-chain D and J segments, then join a V segment to the DJ rearrangement.
RAG1 and RAG2 must be active during these heavy-chain recombination steps.

52:38 A Functional Heavy Chain Triggers Allelic Exclusion

A B cell has two heavy-chain loci, one on each inherited copy of chromosome 14; D-to-J rearrangement occurs on both chromosomes.
The first successful V-to-DJ rearrangement can produce a heavy-chain protein, which is displayed to test whether it can support signaling.
A successful heavy chain triggers allelic exclusion: further heavy-chain rearrangement is shut down so the cell does not express two different heavy chains.
The signal turns off RAG and makes the heavy-chain chromatin less accessible, preventing the second allele from completing rearrangement.

1:01:43 Failed Heavy-Chain Rearrangement Gets a Second Allele Attempt

Junctional insertions that are not multiples of three can cause a frameshift and introduce a stop codon, preventing production of a functional heavy chain.
If the first chromosome fails to make a signaling-capable protein, no successful receptor signal shuts down RAG, so the other chromosome can attempt rearrangement.
If both heavy-chain alleles fail, the developing B cell does not receive the needed survival signal and dies.

1:07:35 The Pre-B-Cell Receptor Tests Heavy Chains with a Surrogate Light Chain

A heavy chain cannot be properly tested on its own because immunoglobulin normally requires both heavy and light chains.
Pro-B and pre-B cells produce VpreB and λ5, which together form a surrogate light chain that stabilizes the heavy chain at the cell surface.
The heavy chain plus surrogate light chain forms the pre-B-cell receptor, which defines the pre-B-cell stage.
Adjacent pre-B-cell receptors can cross-link through long receptor tails, allowing signaling without antigen or a completed light chain.

1:12:02 Successful Pre-B Cells Survive, Proliferate, and Prepare for Light Chains

Pre-B-cell receptor signaling supports survival, shuts off further heavy-chain rearrangement, and stops surrogate-light-chain transcription.
A successful pre-B cell proliferates while RAG is off, reducing the risk of DNA damage during DNA replication.
Proliferation creates progeny that can each later make a different light chain to pair with the validated heavy chain.
RAG is turned back on after this proliferative phase; light-chain rearrangement begins in the next lecture.

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.