14.7 Adaptive Humoral Immunity: B Cells & Immunoglobulins

14.7 Adaptive Humoral Immunity: B Cells & Immunoglobulins

Adaptive Humoral Immunity: B Cells and Immunoglobulins

Humoral immunity is mediated by B lymphocytes and their secreted effector proteins, antibodies (immunoglobulins). B cells originate and mature in the bone marrow, undergoing somatic recombination of gene segments (V-D-J for heavy chains, V-J for light chains via RAG-1 and RAG-2 recombinases) to generate a unique B-Cell Receptor (BCR). Immature B cells that bind multivalent self-antigens with high avidity in the marrow undergo negative selection via clonal deletion (apoptosis) or receptor editing (secondary light-chain rearrangements), establishing central B-cell tolerance. Mature, naive B cells exit to secondary lymphoid tissues expressing membrane-bound IgM and IgD.

Antibody Structural Architecture:
- Heterotetramer: Two identical Heavy (H) chains (~50 kDa each) and two identical Light (L) chains (~25 kDa each).
- Disulfide Bonds: Covalent interchain disulfide bridges link H to L, and H to H at the flexible hinge region.
- Fab Fragment (Fragment Antigen-Binding):
  Contains Variable domains (VH and VL) and first Constant domains (CH1 and CL).
  Hypervariable Loops (Complementarity-Determining Regions: CDR1, CDR2, CDR3) dictate antigen specificity.
- Fc Fragment (Fragment Crystallizable):
  Composed of CH2 and CH3 (and CH4 in IgM/IgE) constant domains.
  Dictates antibody isotype, fixes complement (C1q binding site on CH2), and binds Fc receptors on phagocytes/NK cells.

Effector Mechanisms of Secreted Antibodies:
1. Neutralization: Fab binds viral surface spikes or bacterial toxins, sterically blocking host cell receptor entry.
2. Opsonization: Fab binds pathogen surfaces; exposed IgG Fc domains bind Fc-gamma-RI on macrophages, driving phagocytosis.
3. Agglutination: Multivalent antibodies (e.g., pentameric IgM) cross-link particulate antigens, clumping them for clearance.
4. Complement Activation: Classical pathway initiation via C1q binding to IgG or IgM Fc domains.

B-Cell Activation, Somatic Hypermutation & Isotype Switching

• T-Dependent B-Cell Activation: Protein antigens bind BCRs on naive B cells in lymphoid follicles. The B cell internalizes the BCR-antigen complex, degrades the protein, and displays antigenic peptides on surface MHC Class II molecules. A primed follicular helper T cell ($T_{fh}$) recognizes the peptide-MHC II complex via its TCR and delivers crucial costimulatory signals: T-cell CD40L engages B-cell CD40, and the T cell secretes polarizing cytokines (IL-4, IL-21).
• The Germinal Center Reaction:
  • Somatic Hypermutation (SHM): Proliferating centroblasts in the dark zone activate Activation-Induced Cytidine Deaminase (AID), which deaminates cytosine to uracil in immunoglobulin variable $V(D)J$ exons. Error-prone DNA repair generates point mutations at a rate $10^6$ times higher than background. Centrocytes with mutated BCRs compete for limited antigen on follicular dendritic cells (FDCs) in the light zone; only those with increased binding affinity receive survival signals (Affinity Maturation).
  • Class Switch Recombination (CSR): Under the direction of T-cell cytokines, AID targets switch (S) regions upstream of constant heavy ($C_H$) genes. Double-strand DNA breaks and non-homologous end joining (NHEJ) excise the intervening $C_\mu$ and $C_\delta$ segments, juxtaposing the unchanged $V(D)J$ variable exon to a downstream constant gene ($C_\gamma, C_\alpha, \text{ or } C_\epsilon$). The B cell preserves its antigen specificity while switching its isotype from IgM to IgG, IgA, or IgE.

The Five Immunoglobulin Isotypes

1. IgM: Secreted as a pentamer joined by a polypeptide J chain (possessing 10 antigen-binding sites); monomeric on naive B-cell surfaces. It is the primary antibody produced during the initial, early primary immune response. Due to high avidity, it is the most potent activator of the classical complement cascade and an exceptional agglutinin. Cannot cross the placenta.
2. IgD: Exists as a monomer on naive B-cell membranes alongside IgM; functions primarily as an antigen receptor in B-cell activation. Extremely low serum levels.
3. IgG: Most abundant serum immunoglobulin (~75%–80% of total plasma antibodies; half-life ~21–28 days). Exists as a monomer with four subclasses (IgG1–4). The primary antibody of the rapid, high-affinity secondary immune response. It is the only antibody capable of crossing the human placenta (via neonatal Fc receptors, FcRn), conferring passive immunity to the developing fetus and neonate. Potent opsonin, neutralizer, and mediator of ADCC.
4. IgA: Predominant antibody in mucosal secretions (saliva, tears, colostrum/breast milk, tracheobronchial and gastrointestinal mucus). Secreted as a dimer joined by a J chain and wrapped by a Secretory Component (derived from the polymeric immunoglobulin receptor, pIgR, on epithelial cells), which shields IgA from intraluminal proteolytic digestion. Prevents microbial attachment and colonization on mucosal surfaces.
5. IgE: Monomer with an extra constant domain ($C_H4$). Lowest serum concentration. Binds with extraordinarily high affinity ($K_d \approx 10^{-10}\text{ M}$) to $Fc\epsilon RI$ on mast cells and basophils. Mediates Type I immediate hypersensitivity (allergic asthma, anaphylaxis) and defense against multicellular parasitic helminths.

The Five Immunoglobulin Isotypes

Isotype is determined by the heavy-chain constant region and dictates where the antibody goes and what it does after binding antigen.