B cells

Question 1

What is the B cell receptor (BCR) made from?

Answer 1

The same genes that encode antibodies (immunoglobulins, Ig). The BCR is essentially a membrane-bound antibody.

Question 2

What are the two classes of immunoglobulin light chains?

Answer 2

Lambda (λ) and Kappa (κ).

Question 3

Name the five immunoglobulin isotypes and their corresponding heavy chains.

Answer 3

IgG (γ), IgM (μ), IgA (α), IgE (ε), IgD (δ).

Question 4

Which isotypes have 5 Cₕ domains vs 4?

Answer 4

5 domains: IgM and IgE. 4 domains: IgG, IgD, IgA.

Question 5

What is the serum form of IgM? What forms can IgA take?

Answer 5

IgM is always a pentamer in serum. IgA can be monomers, dimers, or trimers.

Question 6

What are the four subclasses of IgG?

Answer 6

IgG1, IgG2a, IgG2b, and IgG3.

Question 7

What is class switching (CSR)?

Answer 7

A given Ig changes its heavy chain constant region (isotype) while keeping the same antigen-binding (variable) region — e.g., IgM → IgG.

Question 8

State the 5 principles of clonal selection theory.

Answer 8

1. Each lymphocyte expresses one receptor type (pre-existing diversity). 2. Antigen only binds cells with sufficient affinity. 3. Binding triggers activation and clonal proliferation. 4. Clones differentiate into effector cells. 5. Self-reactive cells are eliminated → tolerance.

Question 9

How does clonal selection explain self-tolerance?

Answer 9

Lymphocytes with receptors that bind self too strongly are eliminated during development.

Question 10

What is somatic diversification?

Answer 10

The observed Ig repertoire is generated from a limited number of gene segments (V, D, J) that rearrange during B cell development to produce the variable regions of Ig chains.

Question 11

What enzyme catalyses V(D)J recombination and what does TdT do?

Answer 11

RAG1 and RAG2 perform the recombination. TdT (terminal deoxynucleotidyl transferase) adds random N-nucleotides at junctions, increasing diversity.

Question 12

What happens experimentally if RAG genes are knocked out?

Answer 12

No T cells AND no B cells are produced — useful to study lymphocyte-dependent processes.

Question 13

Where do B-1 cells originate compared to B-2 cells?

Answer 13

B-1 cells arise during embryogenesis (yolk sac / foetal liver). B-2 cells are bone marrow derived throughout adult life.

Question 14

Where do B-1 cells home after development?

Answer 14

Predominantly to the peritoneal cavity, and to a lesser extent the pleural cavity, via CXCL13 signalling.

Question 15

How do B-1 cells maintain their numbers?

Answer 15

They are self-renewing throughout life — little/no contribution from adult bone marrow.

Question 16

What are the two subsets of B-1 cells?

Answer 16

B-1a and B-1b.

Question 17

What antibodies do B-1 cells predominantly produce and what is special about them?

Answer 17

Predominantly IgM, called natural antibodies — produced without prior exposure to pathogens.

Question 18

What is meant by B-1 cells being ‘self-reactive’ and ‘poly-reactive’?

Answer 18

Their BCR responds to self-antigens (e.g. oxidised lipids, apoptotic cells) and has broad, low-specificity binding across many antigens.

Question 19

List 4 functions of B-1 cell-derived IgM.

Answer 19

1. Early pathogen neutralisation. 2. Complement activation. 3. Opsonisation. 4. Clearance of apoptotic bodies → prevents DAMP accumulation and inflammation.

Question 20

Besides antibody production what else can B-1 cells do?

Answer 20

Present antigen and produce cytokines.

Question 21

Describe the 3 developmental waves of B cell production.

Answer 21

1. Yolk sac/PSp → first primitive B-1 cells. 2. Foetal liver → major haematopoietic organ during fetal development. 3. Bone marrow → takes over at birth, produces B-2 (and some B-1) cells.

Question 22

What is the developmental pathway of a B-2 cell from the earliest progenitor?

Answer 22

HSC → MPP → MPP4 → CLP → Pre-pro-B → Pro-B → Large Pre-BII → Small Pre-BII → Immature B → Mature/Recirculating B → Memory / Plasmablast / Plasma cell.

Question 23

What is the order of heavy vs light chain rearrangement in the BM?

Answer 23

Heavy chain first: D-J then V-DJ rearrangement → μ chain pairs with surrogate light chain (VpreB + λ5) → pre-BCR → RAG pauses → cell proliferates. Then light chain: RAG reactivated → V-J rearrangement → IgM expressed on surface → immature B cell.

Question 24

What is the pre-BCR and what does it signal?

Answer 24

μ heavy chain paired with the surrogate light chain (VpreB + λ5). Signals via Igα/Igβ by surrogate L-chain cross-linking (not antigen-specific) — confirms successful H-chain rearrangement.

Question 25

What is central tolerance in B cells?

Answer 25

Testing of immature B cells in the bone marrow for self-reactivity via two mechanisms: (1) receptor editing and (2) clonal deletion.

Question 26

Explain receptor editing.

Answer 26

The self-reactive light chain is replaced by rearranging new V-J segments. Can repeat until no more L-chain segments are available.

Question 27

What is clonal deletion?

Answer 27

If receptor editing fails, the self-reactive immature B cell undergoes apoptosis.

Question 28

What receptor do B cells need to express to exit the bone marrow and why?

Answer 28

S1PR1 (sphingosine-1-phosphate receptor 1). Binds S1P, which is at high concentration in blood, driving migration into vasculature.

Question 29

What is peripheral tolerance in B cells?

Answer 29

Immature B cells in the periphery that encounter cross-linking antigen before they are functionally mature will undergo apoptosis.

Question 30

What are transitional B cells and what surface marker characterises them?

Answer 30

Immature B cells exiting the BM. They express sIgM but not IgD. There are two stages: T1 (BM→spleen) and T2 (in spleen).

Question 31

What are the two mature B cell fates decided at the T2 stage and what signals drive each?

Answer 31

Follicular (FO) B cell: strong BCR + canonical NF-κB. Marginal Zone (MZ) B cell: weak BCR + canonical NF-κB + Notch2 signalling.

Question 32

What survival signals do T2 B cells require?

Answer 32

Tonic BCR signals + BAFF (B cell activating factor) + non-canonical NF-κB.

Question 33

What are the two consequences of splenectomy for B cells?

Answer 33

1. Loss of transitional B cell maturation and peripheral tolerance checkpoints. 2. Loss of marginal zone B cells → impaired T-independent responses to encapsulated bacteria → increased sepsis risk.

Question 34

What chemokine interaction keeps naïve follicular B cells in the B cell follicle?

Answer 34

CXCR5 on B cells interacting with CXCL13 in the follicle.

Question 35

What chemokine interaction keeps T cells in the T cell zone?

Answer 35

CCR7 interaction with CCL19/CCL21.

Question 36

What happens to B cell chemokine receptor expression after antigen encounter?

Answer 36

B cells upregulate CCR7, allowing them to migrate to the interfollicular region to meet T helper cells.

Question 37

What allows T helper cells to migrate toward B cells at the interfollicular region?

Answer 37

TH cell activation leads to upregulation of CXCR5, allowing them to migrate toward CXCL13 in the follicle boundary.

Question 38

What is co-stimulation and why is it needed for B cell activation?

Answer 38

Additional signals beyond BCR engagement (e.g., CD40-CD40L interaction with T helper cells, cytokines) are required to fully activate a B cell and generate immunity.

Question 39

What cells capture antigen and present it to follicular B cells in lymph nodes?

Answer 39

Specialised macrophages capture particulate antigen from lymph. Follicular dendritic cells (FDCs) efficiently capture and display opsonised antigen.

Question 40

What type of B cells mediate T-independent responses and what does the response look like?

Answer 40

Mainly marginal zone B cells. Produces low-affinity antibody responses and only IgM production.

Question 41

What is a germinal centre (GC)?

Answer 41

A specialised, transient microstructure forming in secondary lymphoid organs (SLOs) where antigen-activated B cells undergo rapid proliferation, somatic hypermutation, and affinity-based selection.

Question 42

What are the two phases of the humoral immune response after antigen encounter?

Answer 42

Phase 1 (Day 3–5): Primary foci → plasmablasts → rapid IgM secretion. Phase 2 (Day 4–7+): GC formation → higher affinity, class-switched antibodies + memory B cells.

Question 43

What is EBI2 and what role does it play in B cell fate after activation?

Answer 43

EBI2 is a receptor (ligand: oxysterol). Cells that upregulate EBI2 move outward toward the subcapsular sinus → become plasmablasts. Cells that downregulate EBI2 move back into the follicle → form the GC.

Question 44

What is the master transcriptional regulator of GC B cells?

Answer 44

BCL6.

Question 45

What are the surface markers distinguishing light zone vs dark zone GC B cells?

Answer 45

Light zone: CXCR4ˡᵒ CD83ʰⁱ CD86ʰⁱ (centrocytes). Dark zone: CXCR4ʰⁱ CD83ˡᵒ CD86ˡᵒ (centroblasts).

Question 46

Describe the cyclic re-entry model.

Answer 46

GC B cells cycle between dark zone (somatic hypermutation + rapid proliferation) and light zone (antigen capture from FDCs + presentation to TFH cells → selection). Selected cells return to DZ; unselected cells die.

Question 47

What happens to a B cell in the light zone that cannot bind antigen or get TFH help?

Answer 47

It undergoes apoptosis (cleared by macrophages in the GC).

Question 48

What is somatic hypermutation (SHM)?

Answer 48

Mutations introduced into the V regions (especially CDRs) of antibody genes in dividing GC B cells — outside the germline — passed to daughter cells. Drives affinity maturation.

Question 49

What enzyme drives both SHM and class switch recombination?

Answer 49

AID — Activation-Induced Cytidine Deaminase.

Question 50

How does AID introduce mutations?

Answer 50

AID converts cytosine (C) → uracil (U) in DNA. When the cell repairs this, random mutations are introduced. In the V region this drives SHM; in switch regions it drives CSR.

Question 51

What are CDRs and why are they important in SHM?

Answer 51

Complementarity Determining Regions — the antigen-binding loops of the V region. They are mutational hotspots for AID, and mutations here most directly affect antigen affinity.

Question 52

What is the outcome of most vs some SHM mutations?

Answer 52

Most mutations → worse binding or frameshifts → apoptosis. Some mutations → improved antigen binding → affinity maturation.

Question 53

How does AID drive class switch recombination (CSR)?

Answer 53

AID targets switch regions upstream of heavy chain constant genes, creates double-strand breaks, loops out intervening DNA, and joins the VDJ region to a new constant region — changing isotype without altering antigen specificity.

Question 54

What must occur before AID can access switch regions?

Answer 54

Transcription through the switch regions — this opens the DNA for AID access.

Question 55

Which cytokines direct switching to which isotypes?

Answer 55

IL-4 → IgG1, IgE. IFN-γ → IgG2a, IgG3. TGF-β → IgG2b, IgA. IL-21 → IgG3, IgG1, IgA. IL-5 → IgG1, IgA.

Question 56

How do vaccines exploit GC biology?

Answer 56

Vaccine antigen activates naïve B cells → triggers GC reaction → generates high-affinity antibodies, memory B cells, and long-lived plasma cells.

Question 57

Why are adjuvants needed for killed/attenuated vaccines?

Answer 57

They boost inflammation to ensure proper immune recognition and activation of naive B cells.

Question 58

How do booster vaccines improve antibody responses?

Answer 58

Boosters re-engage memory B cells, triggering another round of SHM → even higher affinity antibodies.

Question 59

How is the mRNA COVID-19 vaccine delivered and what does it encode?

Answer 59

mRNA encoding the SARS-CoV-2 spike protein, delivered via lipid nanoparticles.

Question 60

How was GC activity confirmed after COVID-19 mRNA vaccination in humans?

Answer 60

Fine needle aspiration of draining lymph nodes detected BCL6+/CD38+ GC B cells, confirming a GC reaction occurred.

Question 61

What is SLE and which cells drive it?

Answer 61

Systemic Lupus Erythematosus — an autoimmune disease driven by autoreactive B cells producing antibodies against self-DNA and nuclear proteins.

Question 62

Why is SLE systemic (affecting many organs)?

Answer 62

DNA is in every cell → immune complexes containing anti-DNA antibodies can deposit in microvasculature throughout the body.

Question 63

What role does TLR7/9 play in SLE pathogenesis?

Answer 63

Self-DNA activates TLR7/9 inside B cells, bypassing normal tolerance checkpoints and promoting autoreactive B cell activation.

Question 64

Why does anti-CD20 (rituximab) work poorly in SLE?

Answer 64

The main pathological cells in SLE are long-lived plasma cells, which do not express CD20 and are therefore not depleted by rituximab.

Question 65

What is multiple sclerosis (MS) and what is the primary immune driver?

Answer 65

An autoimmune attack on CNS myelin causing inflammatory demyelination and neurological deficits. Primarily T cell-mediated.

Question 66

Why does targeting B cells (anti-CD20) help in MS even though it's T cell-driven?

Answer 66

B cells act as antigen-presenting cells: they internalise myelin antigen via BCR → present via MHC II to autoreactive CD4 T cells → amplifying the pro-inflammatory response. Depleting B cells removes this APC function.

Question 67

Why does anti-CD20 spare protective immunity in MS treatment?

Answer 67

It spares long-lived plasma cells (no CD20) and BM precursors, preserving pre-existing antibody responses. The effect is also reversible.