Comprehensive Immune System & Pathogen Control Notes

Innate Defences: Barriers in Plants & Animals

The first protective layer against infection is entirely innate—it is genetically encoded, non-specific, and memory-less. All multicellular organisms deploy three broad categories of barriers:

  1. Physical barriers form structural blockades. In plants these include the waxy cuticle, rigid cellulose cell walls and stomatal closures; in animals, tightly packed keratinised skin, insect exoskeletons and mucous membranes. Remember: skin only functions while intact.

  2. Chemical barriers rely on secreted molecules. Plant leaves exude toxins or secondary metabolites (foxglove’s cardioactive glycosides, tomato anti-digestive factors). Animal chemistry spans lysozyme enzymes in tears/saliva, stomach HCl (pH ≈2\approx2), fatty acids on skin, defensins around sperm, acidic urine & vaginal fluid.

  3. Biological barriers (microbiota) – dense, commensal flora on skin, gut and reproductive tracts compete for space/nutrients and modulate pH; analogous to grass preventing weeds germinating.

If Barriers Fail: The Inflammatory Cascade

Tissue damage or pathogen entry sparks a rapid, non-specific inflammatory response that is present from birth:
• Damaged cells release cytokines (interleukin-1, IL-6, TNF-α) acting as chemical beacons.
• Mast cells degranulate, releasing histamine which induces vasodilation and vascular permeability; excessive release risks anaphylaxis. Mast cells also emit cytokines to amplify recruitment.
• Platelets aggregate, secreting fibrin to plug breached vessels.
• Neutrophils (first responders) exit blood, phagocytose a few microbes then apoptose, forming pus with macrophage debris.
• Macrophages (‘big eaters’) and dendritic cells engulf pathogens via phagocytosis, fuse phagosomes with lysosomes (lysozyme digestion), clear apoptotic blebs, and present processed antigens on MHCII.
• Eosinophils arrive later, escalate cytokine signalling and release granules effective against multicellular parasites; in allergies they can damage host tissue.
• Natural Killer (NK) cells patrol for self-marker MHCI absence; upon detecting virus-infected or oncogenic cells they release perforin & granzymes to trigger apoptosis.
• Complement proteins (≈30 plasma factors) exit leaky capillaries, opsonise microbes, punch membrane attack complexes and form chemo-attractant gradients.
• Interferons (type I, II, III) secreted by virus-infected cells up-regulate PKR and MHCI in neighbours and recruit NK/macrophages.
A mild cytokine-driven fever (~39∘C39^{\circ}\text{C}) inhibits many bacterial enzymes; antipyretics can blunt immunity. Uncontrolled cytokine release causes lethal ‘cytokine storms’ (1918 flu, SARS, COVID-19).

Antigen, Self vs Non-Self & Antigen Presentation

• Antigen (Ag) means "antibody generator"—any molecule that can trigger immunity.
• Self-antigens (mostly MHCI glycoproteins, absent only on RBCs) are tolerated by the host but immunogenic in another individual.
• Non-self antigens reside on foreign cells, pollen, venoms, etc., provoking responses.
• Macrophages, neutrophils and dendritic cells digest pathogens, load fragments onto MHCII, migrate to lymph nodes and display them to T- & B-lymphocytes—initiating the adaptive arm.

Lymphatic System – Transport & Activation Hub

Interstitial fluid becomes lymph and drains into blind-ended lymph vessels whose one-way valves and skeletal muscle action return it to blood. Primary lymphoid tissue: bone marrow (all leukocyte genesis) and thymus (T-cell maturation, self-reactive deletion). Secondary tissues: lymph nodes (antigen presentation & clonal selection), spleen, tonsils, appendix. Lymphoedema arises when nodes are removed/damaged, impairing drainage and raising infection risk.

Adaptive Immunity – Cellular & Humoral

Upon pathogen escape of innate defences, highly specific, memory-forming responses deploy.

  1. B-Lymphocytes
    • Naïve B-cells need two signals: (1) direct antigen binding to surface antibody receptor; (2) helper-T-cell-derived interleukin. Without the second, they apoptose (peripheral tolerance).
    • Activated B-cells clonally expand → differentiate:
    – Plasma cells – secrete large quantities of a single soluble antibody (IgM first then IgG/IgA/IgE/IgD). Antibodies possess two identical variable arms (antigen-binding), a flexible hinge for agglutination, and a constant Fc base to recruit complement/ phagocytes.
    – Memory B-cells – long-lived sentinels in lymph nodes; upon re-exposure they rapidly yield more plasma cells, underpinning secondary responses and allergy escalation (IgE–mast cell coupling).

  2. T-Lymphocytes
    • Helper T (CD4+) cells recognise MHCII-presented antigen + co-stimulatory cytokine, then proliferate and orchestrate immunity by secreting cytokines that activate B-cells and cytotoxic T-cells.
    • Cytotoxic T (CD8+) cells require antigen plus cytokine; they identify infected/self-altered cells (viral peptides on MHCI or loss of MHCI) and induce apoptosis via perforin–granzyme or Fas–FasL pathways (cell-mediated immunity).
    • Regulatory & memory subsets maintain tolerance and rapid recall.

Types of Pathogens & Disease Concepts

• Cellular pathogens: bacteria (cocci, bacilli, spirilla, vibrio, rickettsiae), fungi (hyphae, opportunistic Candida), protozoa (motile Plasmodium causing malaria), multicellular parasites (worms, arthropods).
• Non-cellular: viruses (DNA/RNA + capsid; obligate intracellular), prions (misfolded proteins causing transmissible spongiform encephalopathies—CJD, BSE).
• Emerging/re-emerging infections driven by zoonosis, mutation, travel, vaccine decline, antimicrobial resistance (e.g., measles resurgence, SARS-CoV-2 pandemic). Epidemiology classifies endemic/epidemic/pandemic and analyses R0R_0 transmission numbers.
• Aboriginal and Torres Strait Islander Peoples suffered catastrophic mortality post-1788 due to novel European pathogens (smallpox, TB, influenza) meeting immunologically naïve communities; loss compounded by dispossession and limited access to Western or traditional medicines.

Transmission & Control Measures

Direct spread: touch, droplets, airborne suspension, body fluids, faecal-oral. Indirect: vectors (mosquitoes, ticks). Prevention hierarchy: personal hygiene (soap lowers surface tension), safe food storage (below 4∘C4^{\circ}\text{C} / above 65∘C65^{\circ}\text{C}), potable water, vaccination, biosecurity (quarantine, thermal scanners, bans on plant/animal imports). Outbreak management: identify pathogen (culture, Gram stain, susceptibility tests, PCR sequencing), isolate/quarantine, contact tracing, vector reduction, public education, sentinel monitoring.

Immunological Memory & Immunity Categories

Active immunity = host makes memory cells; Passive = ready-made antibodies supplied.
• Natural active: infection recovery.
• Artificial active: vaccination (antigen ± adjuvant; live attenuated or sub-unit; boosters renew memory).
• Natural passive: maternal IgG via placenta & IgA/IgM in milk.
• Artificial passive: antiserum/antivenom (equine/rabbit antibodies).
Herd immunity threshold =1−1R<em>0=1-\frac{1}{R<em>0}; e.g. measles R</em>0≈12!–!18R</em>0\approx12!–!18 → ≥94%\ge 94\% coverage.

Vaccination Programs & R₀ Dynamics

National schedules (Australia’s NIP) give free childhood & booster shots. High virulence often correlates with lower mobility; incubation periods allow stealth spread (measles contagious 1–4 d pre-rash). Social distancing lowers effective ReR_e below 11.

Immunotherapy & Monoclonal Antibodies

Cancer and autoimmune conditions can be tackled by tuning immunity:
• Checkpoint inhibitors block PD-1/PD-L1 or CTLA-4 “off-switches”, re-awakening cytotoxic T-cells against tumours.
• CAR-T therapy engineers patient T-cells with chimeric antigen receptors via CRISPR; massive lab expansion and reinfusion targets leukemias, lymphomas.
• Cancer vaccines (e.g., HPV L1 protein, melanoma peptides) prophylactically or therapeutically elicit tumour-specific responses.
• Monoclonal antibodies (mAbs) – hybridoma-derived identical IgGs bind unique tumour antigens or block cytokines (e.g., anti-TNF-α for rheumatoid arthritis, anti-B-cell CD20 mAb).
Production: inject antigen into mouse → harvest spleen plasma cells → fuse with immortal myeloma → select hybridoma → culture large batches.
• Autoimmune therapy: TNF-α inhibitors curb inflammation; CAR-T or mAbs suppress autoreactive B-cells or antigen-presenting cells.

Key Numerical & Formulae References

• Stomach acid pH≈2\text{pH}\approx2; normal body 37∘C37^{\circ}\text{C}; febrile 39∘C39^{\circ}\text{C} upper safe 40∘C40^{\circ}\text{C}.
• Herd immunity threshold H<em>t=1−1R</em>0H<em>t=1-\dfrac{1}{R</em>0}.
• Vaccine cold chain: maintain 2∘C2^{\circ}\text{C}–8∘C8^{\circ}\text{C}.

Ethical & Practical Considerations

• Antipyretic over-use may hinder immune clearance.
• Phage therapy offers antibiotic alternatives but requires precise bacterial ID.
• Immunosuppressive drugs (cyclosporin, tacrolimus) prevent graft rejection but elevate infection risk.
• Animal-derived antisera and monoclonals raise welfare debates; plant-based expression systems are emerging.
• Equitable vaccine access safeguards herd immunity; misinformation endangers vulnerable cohorts.

Quick Revision Points

• Innate = immediate, broad, no memory; adaptive = delayed, specific, memory.
• Physical, chemical, biological barriers buy time for inflammation.
• Cytokines co-ordinate every stage; too many cause storms.
• Antigen presentation on MHCII triggers B/T differentiation; MHCI labels “self”.
• Lymph nodes = training grounds; bone marrow & thymus = origination/maturation.
• Vaccines safely ‘teach’ without disease; boosters refresh; herd immunity protects the few.
• Immunotherapy harnesses or redirects immunity against malignancy or misdirected autoimmunity.