W5 - B Cells & Antibodies

What bone marrow stromal factors and cytokines drive early B-cell progenitor survival, retention, and commitment?

• FLT3 / FLT3 Ligand: Early progenitor differentiation.

• CXCL12: Retains early progenitors in specific bone marrow niches via CXCR4.

• Kit / SCF (Stem Cell Factor): Pro-B cell proliferation and survival.

• IL-7 / IL-7 Receptor: Essential driving factor for B-cell commitment and progression.

What are the heavy and light chain gene statuses during the Early Pro-B and Late Pro-B stages?

• Early Pro-B: Heavy chain is rearranging D–J; Light chain is Germline.

• Late Pro-B: Heavy chain is rearranging V–DJ; Light chain is Germline.

What molecular complex forms the Pre-BCR during the Pre-B cell stage?

A rearranged mu heavy chain transiently paired with surrogate light chains (VpreB + lambda5), alongside Ig-alpha (CD79a) and Ig-beta (CD79b).

What surface immunoglobulins are expressed on Immature B cells vs. Mature Naïve B cells?

• Immature B Cell: Expresses surface IgM only.

• Mature Naïve B Cell: Co-expresses surface IgM and IgD via alternative mRNA splicing.

What happens during B-cell Central Tolerance in the Bone Marrow?

Immature B cells expressing surface IgM are tested against self-antigens. High-affinity self-reactive clones undergo negative selection via clonal deletion, receptor editing, or anergy.

What are the recognition and signalling components of the B-Cell Receptor (BCR) complex?

• Recognition: Membrane-bound immunoglobulin (mIgM or mIgD).

• Signalling: Disulfide-linked heterodimer of Ig-alpha (CD79a) and Ig-beta (CD79b) containing cytoplasmic ITAM motifs.

What is the sequential phosphorylation cascade following BCR antigen cross-linking?

• Src-family kinases (Blk, Fyn, Lyn) phosphorylate ITAMs on Ig-alpha/Ig-beta.

• Syk kinase binds phosphorylated ITAMs and becomes activated.

• Syk phosphorylates adaptor protein SLP-65 (BLNK).

• Btk is recruited to activate PLC-gamma 2; PI3K activates the Akt cell survival pathway.

• Vav activates WASp, driving actin polymerization for antigen internalization.

What proteins form the B-Cell Co-Receptor Complex and how does it lower the activation threshold?

Composed of CD19, CD21 (CR2), and CD81 (TAPA-1). Complement-coated antigens (C3dg) cross-link BCR with CD21, driving CD19 phosphorylation by Src kinases to recruit PI3K and strongly amplify signaling.

How do Thymus-Independent (TI) and Thymus-Dependent (TD) antigens differ in terms of antigen nature and T-cell requirements?

• TI Antigens: Non-protein structures (e.g., bacterial LPS, polymeric capsular polysaccharides); no T-cell help required.

• TD Antigens: Soluble or particulate protein antigens; require Tfh cell help (MHC-II:TCR, CD40:CD40L, IL-21/IL-4).

How do TI vs. TD activation outcomes compare in isotype production, somatic hypermutation, and immunological memory?

• TI: Produces mostly primary IgM, minimal/no somatic hypermutation, and poor/no memory.

• TD: Produces high-affinity switched isotypes (IgG, IgA, IgE), extensive somatic hypermutation, and robust, long-lived memory B cells and plasma cells.

What chemokines and receptors control resting T and B cell positioning in secondary lymphoid organs?

• T-Cell Zones (PALS/Paracortex): Stromal cells secrete CCL21, attracting CCR7+ resting T cells.

• B-Cell Zones (Follicles): Follicular Dendritic Cells (FDCs) secrete CXCL13, attracting CXCR5+ resting B cells.

How do activated T and B cells relocate to the interfollicular (T–B border) region for antigen presentation?

Activated B cells downregulate follicular retention signals and upregulate CCR7 and EBI2 (receptor for 7alpha,25-HC); activated T cells upregulate CXCR5. Both migrate toward the T–B boundary.

What dictates whether an activated B cell becomes a Primary Focus Plasmablast vs. enters a Germinal Centre?

• Primary Focus Fate: B cells maintaining high EBI2 move extra-follicularly to become short-lived plasmablasts secreting rapid IgM (~10-day lifespan).

• Germinal Centre Fate: B cells interacting with Bcl-6+ Tfh cells downregulate EBI2 and migrate into the follicle center.

What are the key structural and functional differences between the Germinal Centre Dark Zone and Light Zone?

• Dark Zone: Densely packed centroblasts; CXCR4+ (responds to CXCL12 gradient); primary site of Somatic Hypermutation (SHM) and rapid proliferation.

• Light Zone: Centrocytes, FDCs, and Tfh cells; CXCR5+ CXCR4-; primary site of Selection and Class Switch Recombination (CSR).

What enzyme initiates Somatic Hypermutation (SHM) and Class Switch Recombination (CSR), and what regions do they alter?

Activation-Induced Cytidine Deaminase (AID).

• SHM: Introduces point mutations in variable regions (V_H and V_L CDRs) to alter affinity.

• CSR: Replaces heavy-chain constant region genes (C_mu → C_gamma, C_alpha, C_epsilon) to alter effector function without changing antigen specificity.

Which specific cytokines drive Class Switching to IgE/IgG1, IgG2a/IgG3, and IgA?

• IL-4: Drives switching to IgE and IgG1.

• IFN-gamma: Drives switching to IgG2a and IgG3.

• TGF-beta: Drives switching to IgA.

Which is the only antibody isotype capable of placental transfer, and which is predominant in mucosal secretions?

• Placental Transfer: IgG (specifically IgG1, IgG3, IgG2).

• Mucosal Secretions: IgA (dimer with J-chain & Secretory Component, transported via pIgR).

How do IgM and IgG differ in their mechanisms of Classical Complement Activation?

• IgM: Pentameric IgM changes from 'planar' to 'staple' configuration upon binding antigen, allowing a single IgM staple to bind the C1q complex.

• IgG: Requires multiple closely spaced surface-bound monomeric IgGs to form an array that can recruit C1q.

What are the functions of Fc-gamma-RI (CD64), Fc-gamma-RIIB (CD32B), and Fc-gamma-RIII (CD16)?

• Fc-gamma-RI (CD64): High-affinity IgG1/IgG3 receptor on macrophages/neutrophils; mediates phagocytosis and respiratory burst.

• Fc-gamma-RIIB (CD32B): Contains an ITIM motif; delivers inhibitory negative feedback to B cells and mast cells.

• Fc-gamma-RIII (CD16): Low-affinity receptor on NK cells; triggers Antibody-Dependent Cell-Mediated Cytotoxicity (ADCC) via perforin/granzymes.

What receptor and isotype drive immediate degranulation in allergic responses and anti-parasite immunity?

IgE bound to high-affinity Fc-epsilon-RI on tissue mast cells and basophils; antigen cross-linking triggers release of histamine, heparin, and vasoactive mediators.

What are the key biomarkers and lifespans that distinguish Plasmablasts, Plasma Cells, and Memory B Cells?

• Plasmablast: B220^low, CD138+; short-lived (~10 days).

• Plasma Cell: B220-, CD138+; long-lived (months to decades in bone marrow niches).

• Memory B Cell: Isotype-switched surface Ig (IgG+/IgA+), CD27+; long-lived (decades).

Match these key chemokines with their target receptors and biological effects: CXCL12, 7alpha,25-HC, and CXCL13.

• CXCL12 → CXCR4: Retains early progenitors in bone marrow; drives centroblasts into the Germinal Centre Dark Zone.

• 7alpha,25-HC → EBI2: Directs activated T and B cells to the T–B border and outer follicle.

• CXCL13 → CXCR5: Directs resting naive B cells into primary lymphoid follicles and Light Zone centrocytes.

Where do B lymphocytes originate and complete their primary antigen-independent maturation in mammals?

They originate from hematopoietic stem cells in the bone marrow, which is also the primary site where they undergo V(D)J recombination and central tolerance (negative selection against self-reactivity).

What is the fundamental difference between antigen-independent and antigen-dependent B-cell development stages?

• Antigen-Independent Phase: Occurs in the bone marrow, involving gene rearrangement (V(D)J recombination) to produce a diverse repertoire of naive B cells expressing unique membrane-bound antibodies (BCRs) without encountering foreign antigen.

• Antigen-Dependent Phase: Occurs in secondary lymphoid organs (lymph nodes/spleen) after naive B cells encounter their specific antigen, leading to clonal expansion, somatic hypermutation, and class-switch recombination.

What are the structural components that form a standard monomeric immunoglobulin (antibody) molecule?

Two identical heavy chains and two identical light chains linked together by flexible disulfide bonds, forming a symmetric "Y"-shaped structure divided into distinct variable (V) and constant (C) regions.

What is the functional role of the Fab region versus the Fc region of an antibody?

• Fab Region (Fragment antigen-binding): Formed by the paired variable domains of the heavy and light chains (V_H and V_L); responsible for specifically recognizing and binding the target antigen.

• Fc region (Fragment crystallizable): Formed by the constant domains of the heavy chains; interacts with host Fc receptors on immune cells and complement proteins to mediate effector functions (e.g., opsonization, ADCC, complement activation).

What is Somatic Hypermutation (SHM), and in which specialized microenvironment does it occur?

A cellular process where point mutations are introduced at a high frequency into the variable regions of immunoglobulin genes within activated B cells, occurring in the germinal centers of secondary lymphoid organs.

How does Affinity Maturation result from somatic hypermutation and clonal selection?

B cells whose mutated B-cell receptors (BCRs) bind antigen with higher affinity are preferentially selected and survival-signaled by follicular dendritic cells (FDCs) and T follicular helper (T_FH) cells, driving the progressive increase in overall antibody binding affinity during an immune response.