Spotlight #40 – Immunology & Infectious Disease: Immune Architecture, Vaccine Mechanisms and Epidemic Dynamics

Immunology sits at the intersection of molecular biology, evolutionary theory, and systems medicine. The immune system must solve a combinatorial problem of staggering scale: recognise an essentially unlimited universe of possible antigens using a finite genome, triggering responses against pathogens while remaining tolerant to 30 000 self-proteins. The solution involves stochastic gene rearrangement (generating >10¹⁸ unique receptors), clonal selection, and multilayer feedback control. Mathematical epidemiology then scales these molecular events to population level.

1. Innate Immunity

Innate immunity is the first line of defence, operating within seconds to minutes of pathogen encounter. It relies on a limited repertoire of pattern recognition receptors (PRRs) that detect conserved microbial structures — so-called pathogen-associated molecular patterns (PAMPs) — that are absent from host cells. Because PAMPs are essential for microbial survival and highly conserved across species, they cannot easily be mutated to evade detection.

Toll-like receptors (TLRs) — membrane & endosomal:
  TLR4: lipopolysaccharide (LPS) from gram-negative bacteria
  TLR3/7/8: single-stranded and double-stranded RNA (viral)
  TLR9: unmethylated CpG DNA (bacterial/viral)
  Downstream signalling: MyD88 → NF-κB → pro-inflammatory cytokines
  Trif pathway: IRF3 → IFN-β (anti-viral type I interferon)

cGAS-STING pathway (cytosolic DNA sensing):
  cGAS detects dsDNA in cytoplasm → synthesises cGAMP
  STING → IRF3+NF-κB → IFN-β + TNF + IL-6
  Cancer immunotherapy target: STING agonists for tumour microenvironment

Complement cascade:
  Classical: C1q binds antibody·antigen complex
  Lectin: MBL binds mannose on pathogens
  Alternative: spontaneous C3 hydrolysis (always active at low level)
  All converge at C3 convertase: C3 → C3a (anaphylatoxin) + C3b (opsonin)
  Terminal: C5–C9 → membrane attack complex (MAC) pore

Interferons (IFN):
  Type I: IFN-α/β — antiviral state in neighbouring cells
    Induce ~300 interferon-stimulated genes (ISGs) via JAK-STAT1/2-ISGF3
    Upregulate MHC I for enhanced cytotoxic T-cell killing
  Type II: IFN-γ — from NK cells and T-helper 1 cells
    Activates macrophages: increased phagocytosis + ROS + nitric oxide

Natural killer (NK) cells:
  "Missing self" rule: kill cells lacking MHC I (= stressed, infected, or tumour)
  Activating: NKG2D (stress ligands MICA/MICB) + DNAM-1
  Inhibitory: KIR/CD94-NKG2A → check MHC I expression
  Balance determines kill/no-kill outcome (integration of signals)
          

2. Adaptive Immunity: T Cells

The adaptive immune system generates diversity through V(D)J recombination — a RAG1/2 endonuclease randomly cuts and rejoins gene segments from the Variable, Diversity, and Joining regions of T-cell receptor and immunoglobulin loci. The resulting receptor repertoire is estimated to encompass 10¹⁵–10¹⁸ unique specificities before any antigen exposure, then undergoes clonal selection when a cognate antigen is encountered.

V(D)J recombination diversity:
  TCRα chain: ~50 Vα × 61 Jα = ~3 000 combinations
  TCRβ chain: ~52 Vβ × 13 Dβ × 13 Jβ = ~8 800 combinations
  Combinatorial: 3 000 × 8 800 = 2.6 × 10^7
  Junctional diversity (P- and N-nucleotide addition at CDR3):
    +10^9 additional → total TCR space >10^15 unique receptors
    Compare: ~10^7 T cells per individual at time point

MHC antigen presentation:
  MHC class I (HLA-A/B/C): presents intracellular peptides (8–10 aa)
    → to CD8⁺ cytotoxic T cells (CTL)
    Peptide loaded in ER via TAP transporter, β₂-microglobulin assists folding
  MHC class II (HLA-DR/DQ/DP): presents extracellular peptides (13–25 aa)
    → to CD4⁺ T-helper cells
    Loading in endosome; CLIP peptide displaced by HLA-DM

T-cell differentiation from naive precursors:
  Signal 1: TCR + pMHC (antigen)
  Signal 2: CD28 + B7 (co-stimulation; absence → anergy)
  Signal 3: cytokine milieu → fate
    IL-12 + IFN-γ → Th1 (intracellular killers, macrophage activation)
    IL-4           → Th2 (helminth; IgE)
    IL-6 + TGF-β   → Th17 (neutrophil recruitment; barrier defence)
    TGF-β alone    → T-reg (FoxP3⁺; peripheral tolerance)

CD8⁺ CTL killing mechanism:
  Perforin (pore-forming) + Granzymes A/B (serine proteases) → caspase 3 apoptosis
  FasL + Fas (death receptor) pathway
  Kill efficiency: ~10 min per target cell; serial killing observed
          

3. Humoral Immunity: B Cells and Antibodies

B cells produce antibodies — soluble proteins that bind antigens with high specificity. After initial activation, B cells enter germinal centres in lymph nodes where they undergo somatic hypermutation at a rate 10⁵-times higher than the genomic baseline, with selection for affinity to antigen. This iterative Darwinian process, affinity maturation, produces antibodies with K_d values in the picomolar range.

Immunoglobulin structure:
  Basic unit: two heavy + two light chains (Y-shape)
  Variable regions (VH + VL): antigen-binding site (paratope)
  Complementarity-determining regions (CDR1/2/3): hypervariable loops contact antigen
  Constant region (Fc): effector function (opsonisation, complement, ADCC)

Antibody classes (isotypes) — defined by heavy chain constant region:
  IgM:  pentamer (J-chain); first response; low affinity, high avidity; activates complement
  IgG:  monomer (IgG1–4); most abundant serum Ab; crosses placenta; ADCC
  IgA:  dimer (secretory J-chain + SC); mucosal immunity; breast milk
  IgE:  monomer; binds FcεRI on mast cells/basophils; allergy + anti-parasite
  IgD:  mainly BCR on naive B cells; signalling role

Class switch recombination (CSR):
  AID (activation-induced cytidine deaminase) deaminates C→U in switch regions
  Excises constant region → IgM→IgG/IgA/IgE depending on cytokine signal:
    IL-4 → IgE;  TGF-β → IgA;  IFN-γ → IgG2a (mouse)/IgG1 (human)

Germinal centre affinity maturation:
  Somatic hypermutation rate: ~10^-3 per bp per division (vs ~10^-9 genomic)
  Selection: centrocytes with higher BCR affinity for FDC-displayed antigen survive
  Affinity gain: 10–1000× over ~3 weeks of GC reaction
  Outputs: long-lived plasma cells (Bone Marrow) + memory B cells

Monoclonal antibodies (mAb):
  Therapeutic IgG: trastuzumab (HER2+), pembrolizumab (PD-1), rituximab (CD20)
  Fc engineering: half-life extension (YTE mutation), ADCC optimisation
  Bispecific: blinatumomab = anti-CD3 × anti-CD19 (T-cell engager)
          

4. Vaccines and Immune Memory

A vaccine pre-arms the immune system with the memory of an antigen it has never encountered as a live pathogen. The four main platforms differ in how they deliver antigen, the quality of the immune response they elicit, and their manufacturing complexity.

Inactivated vaccines (e.g. influenza, IPV):
  Killed pathogen → preserves surface proteins
  Innate adjuvant activity from PAMPs (flagellin, LPS) in preparation
  Typically requires 2–3 doses + boosters; weaker cell-mediated response

Live-attenuated vaccines (MMR, yellow fever, varicella):
  Passaged virus loses virulence but maintains immunogenicity
  Single dose often sufficient; strongest and longest T-cell + B-cell response
  Contraindicated in immunocompromised (risk of reversion to virulence)

Subunit / protein vaccines (HepB, HPV Gardasil):
  Recombinant protein antigen + adjuvant (alum/AS04/AS01)
  AS01 (shingrix): MPL + QS-21 → activates TLR4 + induces IL-12 → strong Th1
  No risk of infection; requires adjuvant to overcome poor immunogenicity

mRNA vaccines (SARS-CoV-2 BNT162b2/mRNA-1273):
  LNP-encapsulated modified mRNA (pseudouridine replaces U → evades TLR7/8)
  Translated by host ribosomes → endogenous antigen → MHC I + II presentation
  Generates both CD8⁺ CTL and CD4⁺ Tfh → germinal centres → neutralising IgG
  Turnaround: sequence → first dose in 63 days (Moderna COVID-19 trial record)

Herd immunity threshold (SIR model):
  h_c = 1 − 1/R₀
  Examples:
    Measles:   R₀ ≈ 15 → h_c = 93%
    COVID-19:  R₀ ≈ 3–6 (original) → h_c = 67–83%
    Influenza: R₀ ≈ 1.3 → h_c = 23%
  Effective R₀ with vaccine coverage p_v and VE e:
    R_eff = R₀ (1 − p_v · e)   < 1 required to eliminate
          

5. SIR and SEIR Epidemic Models

Compartmental models partition a population into disease states and describe transitions by differential equations. The basic reproductive number R₀ — the mean number of secondary infections caused by one infected individual in a fully susceptible population — determines epidemic fate: R₀ > 1 leads to exponential growth, R₀ = 1 is the endemic threshold, and R₀ < 1 leads to extinction.

SIR model (Kermack & McKendrick, 1927):
  dS/dt = −β S I / N
  dI/dt =  β S I / N − γ I
  dR/dt =  γ I
  N = S + I + R = const (closed population, no vital dynamics)

  β = effective contact rate [contacts/day × P(transmission)]
  γ = recovery rate = 1/infectious_period [day⁻¹]
  R₀ = β/γ

Epidemic threshold:
  Peak incidence when S = γN/β = N/R₀
  Final attack rate solves: 1 − f = e^(−R₀·f)   (transcendental equation)
  Approximate final size: f ≈ 1 − e^(−R₀)   for small R₀
  Example R₀=3: f ≈ 94% of population eventually infected (no intervention)

SEIR model (adds exposed/latent compartment):
  dS/dt = −β S I / N
  dE/dt =  β S I / N − σ E   (σ = 1/latent_period)
  dI/dt =  σ E − γ I
  dR/dt =  γ I
  R₀ = β/γ  (unchanged; latency shifts timing, not final size)
  Generation time: T_g = T_latent + T_infectious/2

Non-pharmaceutical interventions:
  β is reduced by mask use, social distancing (contact reduction)
  R_eff(t) = R₀ · S(t)/N · [1 − ε(t)]  (ε = intervention efficacy)
  Reproduction number estimated from rt.live or EpiNow2 (Bayesian Rt estimation)

Age-structured SIR:
  Contact matrix C_ij (WHO POLYMOD data): contacts between age group i and j
  β_ij = p_ij × C_ij   (heterogeneous mixing → different R₀ estimates)
  Younger: higher contact rates; older: higher IFR (infection fatality ratio)
          

6. Dysregulated Immunity: Autoimmunity and Immunotherapy

The same mechanisms that generate a vast repertoire of antigen-specific receptors can go wrong in two ways: self-reactive clones may evade deletion (autoimmunity), or the immune system may mount inappropriately strong responses to harmless antigens (allergy). Conversely, tumours exploit immune checkpoints to escape destruction. Modern immunotherapy drugs aim to restore or redirect the response.

Hypersensitivity types (Gell & Coombs classification):
  Type I (IgE-mediated): allergen → IgE on mast cells → degranulation → histamine
    Conditions: anaphylaxis, asthma, urticaria, allergic rhinitis
    Desensitisation: sublingual/subcutaneous immunotherapy (build IgG4 blocking)
  Type II (cytotoxic): IgG/IgM targets cell-surface antigen → CDC/ADCC/phagocytosis
    Examples: autoimmune haemolytic anaemia, Goodpasture syndrome
  Type III (immune complex): soluble IC deposits in vessel walls → complement + neutrophils
    Examples: SLE, serum sickness, vasculitis
  Type IV (cell-mediated, delayed 48–72h): Th1 + CD8 CTL; no antibody
    Examples: contact dermatitis, TB PPD skin test, allograft rejection

Autoimmunity — central & peripheral tolerance failure:
  Central (thymus): negative selection of high-affinity self-reactive T cells
    AIRE transcription factor: expresses peripheral antigens in thymus
    AIRE mutation → APECED (multi-organ autoimmunity)
  Peripheral: T-reg suppression; CTLA-4 competing with CD28 for B7
    Loss of T-reg → spontaneous autoimmunity (FoxP3 knockout = scurfy mouse)
  Molecular mimicry: pathogen epitope resembles self (strep M protein → rheumatic fever)

Checkpoint inhibitors (cancer immunotherapy):
  CTLA-4 blockade: ipilimumab (anti-CTLA-4) → removes co-inhibitory brake on T-cell priming
  PD-1/PD-L1 blockade: pembrolizumab/nivolumab → restores exhausted TIL function
    PD-L1 expressed by tumour cells → dampens CD8⁺ TIL killing → evades immune
  Objective response rates:
    Melanoma (ipilimumab): 20% vs 10% (prior therapy)
    NSCLC (pembrolizumab + chemo): 60% vs 45%
  irAE (immune-related adverse events): ~30% Grade 3–4, managed with corticosteroids
  CAR-T cell therapy: anti-CD19 (tisagenlecleucel), anti-BCMA (idecabtagene)
    Cytokine release syndrome (CRS) graded I–IV; tocilizumab (anti-IL-6R) rescue
          

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