17. B Cell Development and B1

Where are naïve B cells located in lymph nodes, and what is their role before activation?

• Lymph nodes contain an army of naïve B cells—not yet activated and not secreting antibody.

• Their job is to wait for any potential antigen they might recognize.

• They reside in B-cell follicles at the outer (peripheral) regions of the lymph node.

What happens when a B cell is activated?

• Naïve B cell encounters its specific antigen.

• Activation triggers differentiation into a plasma cell.

• Plasma cells secrete large amounts of antibody.

• These antibodies provide protective immunity.

Where do B cells develop and mature?

• B-cell progenitors come from the bone marrow.

• Unlike T cells, B cells do not travel to the thymus.

• All maturation happens in the bone marrow.

What does the “B” in B cell stand for, and how was it discovered?

• “B” does NOT stand for bone marrow.

• It stands for bursa of Fabricius—an organ in birds, not humans.

• B cells were first discovered in chickens by removing the bursa and observing loss of B cells.

• Early B-cell research wasn’t recognized as highly as T-cell work because the role of B cells was not yet understood.

• We now know B cells make antibodies essential for protection.

Why must each B cell express only one B-cell receptor (BCR) specificity?

• B cell must express one BCR sequence → one antigen specificity.

• Prevents a single B cell from making antibodies with mixed specificities.

• If a second specificity recognized self, infection-triggered activation could drive autoantibody production → dangerous self-attack.

• B cells with self-reactive BCRs must be removed.

• Functional B cells must also be able to traffic to lymph nodes and respond to activation signals.

How do B cells generate their B-cell receptors, and what do naïve B cells express?

• BCR/antibody generation uses VDJ recombination (like TCRs).

• Requires rearrangement of heavy chain (analogous to TCR β) and light chain (analogous to TCR α).

• Productive recombination yields an IgM antibody first.

• Unique to B cells: they can later make multiple antibody isotypes.

• Naïve B cells co-express IgM and IgD on their surface—the only stage that expresses both.

What is the overall structure of B-cell development?

• Development occurs through multiple defined stages in the bone marrow.

• Heavy-chain and light-chain genes undergo sequential rearrangements.

• Each stage includes checkpoints determining whether the cell survives or dies.

What does “germline” mean in the context of immunology and VDJ recombination?

• Germline DNA = the unchanged sequence present in sperm or ovum.

• Contains all V, D, and J segments before rearrangement.

• Stem cells retain germline configuration so all descendant B cells can create unique rearrangements.

• If the stem cell rearranged its DNA, all descendant cells would share the same VDJ → no diversity.

• Early stem cells show no rearrangement in heavy or light chains.

What happens during the early pro-B cell stage?

• Early pro-B cells begin heavy-chain recombination.

• First step: D–J joining on the heavy-chain locus.

• B cells attempt D–J joining on both chromosomes (maternal + paternal).

• Two heavy-chain loci = two chances for successful recombination.

What occurs during the late pro-B cell stage?

• After successful D–J joining → cell moves to late pro-B stage.

• Next step: V → DJ recombination (full heavy-chain rearrangement).

• Productive rearrangement = cell survival; non-productive = retry or die.

• Only ~50% of cells successfully complete this step.

How does a B cell handle failed heavy-chain rearrangements?

• If first chromosome’s V→DJ rearrangement fails, the cell tries the second chromosome.

• If the second attempt also fails → apoptosis.

• This system ensures each B cell tries to produce one functional heavy chain, while preventing survival of nonfunctional cells.

What is the first checkpoint in B-cell development after productive VDJ recombination?

• After a productive heavy-chain VDJ, the cell forms a pre-B-cell receptor.

• Uses a surrogate light chain (parallel to surrogate α chain in T cells).

• If surrogate light chain pairs correctly with the heavy chain → successful signaling → survival.

• If the heavy chain cannot pair with a light chain → no signal → death by neglect.

• Ensures the heavy chain is both properly folded and able to form a functional BCR.

What cellular changes occur when a pre-B-cell receptor signals successfully?

• Pre-BCR signaling triggers three events:

  1. Stops RAG transcription.

  2. Destroys existing RAG proteins.

  3. Tightly coils the heavy-chain locus to prevent further rearrangement.

• Prevents recombination of the second heavy-chain allele.

• Guards against producing two different heavy chains, which would give multiple specificities.

What is allelic exclusion in B cells?

• Only one heavy-chain allele completes VDJ recombination.

• The other allele stays germline or partially rearranged.

• Guarantees each B cell expresses only one antibody specificity.

• Essential to avoid mixed or self-reactive specificities.

What happens after a B cell passes the heavy-chain checkpoint?

• The cell undergoes proliferation, similar to T cells.

• Purpose: expand the small number of cells that successfully made a functional heavy chain.

• Produces many daughter cells that will then attempt light-chain rearrangement.

What defines the transition to the large pre-B cell stage?

• Everything from stem cell → early pro → late pro leads up to the large pre-B cell.

• Called “large” because the cell grows before division; after division, they become smaller.

• This stage marks the moment after successful heavy-chain checkpoint.

• “μ” heavy chain refers to IgM, meaning the cell has now produced an IgM heavy chain.

How do small pre-B cells rearrange their light chains, and why is the κ:λ ratio clinically useful?

• Small pre-B cells begin light-chain recombination.

• Two light-chain types: κ (kappa) and λ (lambda).

• Cells always try κ first on both chromosomes.

• Most B cells end up expressing κ:

  • Humans: ~66–75% κ

  • Mice: ~95% κ

• Diagnostic use:

  • B-cell cancers skew κ:λ ratio.

  • κ-tumor → very high κ percentage.

  • λ-tumor → very high λ percentage.

• Successful κ rearrangement → IgM heavy chain (μ) + κ light chain → functional antibody.

What happens if κ light-chain rearrangement fails?

• If κ rearrangement is non-productive, cell switches to λ rearrangement.

• Tries λ on both chromosomes.

• Successful λ rearrangement → B cell expresses λ light chain.

• If both κ and λ fail → apoptosis (cell death).

What is unique about repeated rearrangement attempts in light-chain development?

• Light chains can repeatedly attempt V–J recombination.

• If V–J attempt fails, cell tries another V and another J.

• Can retry across both κ alleles and both λ alleles.

• This high flexibility lets ~85% of cells successfully form a functional light chain.

• Significantly increases BCR diversity and survival.

What happens at the second checkpoint of B-cell development?

• Cell now has a complete BCR (heavy + light chain).

• BCR signaling triggers:

  • Stop RAG transcription

  • Destroy RAG proteins

  • Coil both light-chain loci to block further rearrangement

• Ensures allelic exclusion (only one allele used) and isotypic exclusion (only κ or λ, not both).

• Result: each B cell expresses one unique BCR.

Why does the immune system enforce allelic and isotypic exclusion for light chains?

• Prevents a single B cell from pairing one heavy chain with two different light chains.

• Multiple light chains → multiple specificities, which is dangerous.

• Antibody specificity is dictated by both heavy and light chain variable regions.

• Antibody function (IgM, IgG, IgA, etc.) is determined entirely by the heavy chain.

• Light chain exists to expand diversity and ensure proper antibody structure.

What remains unknown after successfully forming a single B-cell receptor?

• At this point, the B cell has:

  • One BCR sequence

  • One specificity

• But we still don’t know:

  • Whether the BCR binds self-antigens (must be tested).

  • Whether the cell can navigate to lymph nodes and function properly.

• These requirements must be checked in later developmental stages.

Where does negative selection of B cells occur, and why is it less strict than T-cell negative selection?

• B-cell negative selection occurs in the bone marrow.

• Bone-marrow stromal cells + blood flow expose B cells to many self-antigens.

• Only cells that do NOT bind self strongly can exit to the periphery.

• Many self-reactive B cells still escape, because:

  • T cells are already tightly selected in the thymus → self-reactive B cells usually won’t receive T-cell help, so risk is lower.

  • Evolution placed far more pressure on controlling self-reactive T cells than B cells.

How do B cells attempt to fix self-reactivity during development?

• Unlike T cells, B cells can undergo receptor editing.

• When a B cell binds self too strongly, it reactivates light-chain rearrangement.

• Attempts new V–J combinations to change specificity.

• If new light chain removes self-reactivity → cell escapes bone marrow.

• Unique to B cells; T cells simply die instead.

What happens if receptor editing fails to eliminate self-reactivity?

• B cell keeps trying new light-chain rearrangements until no options remain.

• If still self-reactive → apoptosis.

• Likely means self-reactivity is driven by the heavy chain, which can’t be edited at this stage.

Why are some self-reactive B cells allowed to survive, and what happens to them?

• Full deletion of all self-reactive B cells would create holes in immune coverage.

• Viruses could exploit this by mimicking self molecules.

• Some self-reactive B cells escape but become anergic (functionally silenced).

• Anergic B cells:

  • Turn off IgM, keep only IgD on the surface.

  • Require a strong antigen signal to wake up.

• If a virus mimics self and replicates heavily → enough antigen accumulates → anergic B cell reactivates and adapts specificity to target virus more than self.

What remains after negative selection before a naïve B cell is complete?

• After dealing with self-reactivity, the final requirement is migration to lymph nodes.

• Only cells that can leave the bone marrow and home to lymphoid tissues become full naïve B cells.

Why do naïve B cells uniquely express both IgM and IgD?

• B cells normally express only one heavy-chain isotype.

• Exception: naïve B cells express both IgM and IgD.

• IgD’s function is unknown:

  • Humans secrete it, but its role is unclear.

  • Knockout of IgD → immune response becomes strange, but B cells can still function.

  • Knockout of IgM → IgD cannot compensate.

• Something important about IgD, but mechanism unknown → open research question.

What is positive selection for immature B cells, and where does it occur?

• Immature B cells exit bone marrow → enter circulation.

• They undergo positive selection after negative selection (opposite order from T cells).

• Must successfully enter a B-cell follicle via a high endothelial venule (HEV).

• Only cells that can migrate into the follicle survive.

• Failure to access follicle → death within a few days.

• Success → transition to mature B cells that can survive for weeks–months.

How do follicular dendritic cells support final B-cell maturation?

• Follicular dendritic cells (FDCs) secrete BAFF (B-cell activating factor).

• B cell must:

  • Enter follicle via HEV

  • Bind BAFF

• Cells that achieve both (better than competitors) mature into ready-to-activate naïve B cells.

• Mature naïve B cells circulate through lymph nodes, spleen, Peyer’s patches, and blood searching for antigen.

What are the survival statistics and lifespan of immature vs. mature B cells?

• 10–20 million immature B cells exported daily (≈5–10% of total B cell pool).

• Most immature B cells die within days.

• Mature naïve B cells can live up to ~2 months.

• Memory B cells and long-lived plasma cells may persist longer than the lifespan of the organism (decades+).

What is the final “B cell checklist”?

• BCR ✓

• Single BCR sequence ✓

• Non-self-reactive X

• Can migrate to lymph nodes ✓

What comes after generating the naïve B-cell pool?

• Next topic is B-cell activation:

  • How B cells find antigen

  • How they become activated

  • How activated B cells communicate with T cells

  • How they initiate an antibody response

What do B cells ultimately do, and what questions remain?

• B cells protect via antibodies.

• Activation → differentiation into plasma cells → antibody secretion.

• Key questions:

  • How do naïve B cells become activated?

  • How do they produce antibodies?

  • How do antibodies neutralize pathogens?

What distinguishes B1 cells from conventional B2 cells?

• B1 cells are innate-like, similar to γδ T cells.

• Restricted repertoire—recognize limited antigens.

• Produce mainly IgM, little switching; minimal somatic hypermutation.

• Do not require T-cell help.

• Make limited memory.

• B2 cells are the conventional, adaptive B cells (not covered here).

How are B1 cells activated?

• B1 cells bind repetitive carbohydrate antigens (common on bacteria).

• High repetition clusters large numbers of BCRs → strong activation signal.

• Primary location is peritoneal and pleural cavities.

• TLRs often provide additional push.

• B-cell activation requires crossing a signal threshold:

  • Many clustered BCRs or

  • TLR help or

  • T-cell help (for B2 cells)

• Repetitive antigens can activate B1 cells without T-cell help.

What kind of antibodies do B1 cells make, and how effective are they?

• B1 cells mainly produce IgM.

• No somatic hypermutation → low-affinity, “crude” antibodies.

• Sufficient for early defense:

  • Bind broadly

  • Block bacterial attachment

• Not high-quality, refined antibodies like those from T cell–dependent responses.

How are B2 cells activated, and what makes their responses so important?

• B2 cells mediate most adaptive immunity:

  • Protective immunity

  • Autoimmunity

  • Anti-cancer antibody responses

  • Allergic responses

• Activation is T-cell dependent.

• B2 cell process:

  1. B cell binds antigen.

  2. Internalizes & presents it.

  3. T cell confirms antigen match → gives activation signals.

• Activated in secondary lymphoid organs (lymph nodes, spleen, Peyer’s patches).

• Unlike B1 cells, B2 cells never activate alone—require T-cell help.

• T cells provide signals that:

  • Activate the B cell

  • Trigger class-switch recombination (IgM → IgG, IgA, IgE, etc.)

  • Enable somatic hypermutation to improve affinity

  • Select for improved clones (“affinity maturation”)

• Produces memory B cells, essential for long-term protection (vaccines rely on this).

What major B2-cell processes will be covered in following lectures?

• How B2 cells become fully activated

• How they perform class switching to new isotypes

• How affinity maturation improves antibody quality

• What antibodies do to protect against infection

• Continuation of B-cell biology in later lectures