Comprehensive Immunology & Defence System Study Notes

Overview of Human Immunology & Defence

These notes consolidate the information from the transcript into a single, logically-organised reference on the human defence system. All major and minor points, illustrative examples, and mechanistic details are preserved.

Types of Pathogens

1. Viruses – “Non-Living” Infectious Particles

• Structure – nucleic acid core (DNA or RNA) + surrounding protein coat (capsid).

• Relative size – $\approx 50 \times$ smaller than an average bacterium.

• Replication strategy – enter living host cells → viral genome hijacks host biochemistry → mass-produces new virions.

• Pathogenic effect – destruction or functional shut-down of host cells → symptoms arise from tissue damage and immune reaction.

2. Bacteria – Most Numerous Cellular Organisms

• Pathogenic proportion – only a small subset are disease-causing (communicable organisms).

• Cell-wall classification (Gram staining)

  • Gram-positive: Thick peptidoglycan layer, appears purple-blue. Example: Staphylococcus aureus (including MRSA).

  • Gram-negative: Thin peptidoglycan + outer membrane, appears red. Example: Escherichia coli.

  • Clinical relevance – cell-wall type dictates antibiotic susceptibility.

3. Fungi

• Minor threat in animals; notable diseases: athlete’s foot, oral/vaginal thrush.

• Mechanism – extracellular digestion of living cells → tissue destruction.

• Agricultural impact – rapid spread through crops → major economic losses.

4. Protists (Protista)

• Eukaryotic, highly diverse feeding modes.

• Pathogenic subset is parasitic; usually does not integrate its DNA into host genome.

• Transmission

  • Vector-borne: e.g. Plasmodium spp. (malaria) via Anopheles mosquitoes; Trypanosoma spp. (sleeping sickness) via tsetse flies.

  • Water/ingestion: e.g. Entamoeba histolytica → amoebic dysentery.

Transmission Routes of Communicable Diseases

Direct Transmission

• Contact with body fluids: kissing, sexual activity → STDs, bacterial meningitis.

• Skin-to-skin contact: athlete’s foot, ringworm.

• Faeco-oral: diarrhoeal diseases.

• Ingestion: contaminated food/water → amoebic dysentery.

• Inoculation (breach of skin)

  • Sexual micro-trauma → HIV/AIDS.

  • Animal bites → rabies.

  • Needles/punctures → septicaemia.

Indirect Transmission

• Fomites – inanimate objects (bedding, socks, cosmetics) → athlete’s foot, gas gangrene.

• Droplet infection – aerosolised saliva/mucus while speaking, coughing → influenza, tuberculosis.

• Vectors – usually animals (mosquitoes, rat fleas). Water can function as a vector too.

Transmission-Modifying Factors (Examples)

• Population density, hygiene standards, climate/humidity, vector abundance, sanitation infrastructure, vaccination coverage.

Defence Strategy: Three Lines

First Line of Defence – Physical & Chemical Barriers

Skin

• Physical wall; stratified, keratinised epithelium resists penetration.

• Chemical secretions: sweat & sebum → lactic acid, free fatty acids, low pH $\approx 5$.

• Commensal microbiota out-compete pathogens.

Respiratory Tract

1. Sticky mucus traps particulates.

2. Ciliated epithelium escalator clears mucus toward pharynx.

3. Secreted antibodies (IgA) in mucosal fluid.

4. Phagocytosis by resident macrophages.

Digestive Tract

• Stomach acid pH <2 rapidly kills microbes.

• Digestive enzymes (proteases, bile salts) damage membranes.

• Mechanical flushing by peristalsis.

• Normal flora produce bacteriocins in the colon.

• Alkaline pH in small intestine further stresses pathogens.

Eyes

• Continuous washing by tears.

• Tears contain lysozyme (digests peptidoglycan).

Genitourinary Tract

• Urine flow mechanically flushes.

• Urine acidity + lysozyme lethal to many bacteria.

• In females, vaginal lactic acid (from lactobacilli) maintains pH $\approx 4$.

Saliva

• Contains antibacterial enzymes (lysozyme, lactoferrin).

Blood-Clotting Barrier (Wound Repair)

• Cut → platelets contact collagen → release thromboplastin.

• Thromboplastin triggers a coagulation cascade culminating in a fibrin clot.

• Clot seals wound → stops blood loss and blocks pathogen entry.

Immunological Considerations

• Immune cells constantly evaluate:

  1. Self vs. non-self (foreign antigens).

  2. Damaged vs. undamaged self (danger theory).

  3. Tolerance vs. response – sometimes best to ignore (e.g., gut flora).

Innate (Internal) Immunity – Rapid, Non-Specific

Major Cellular Players

• Neutrophils – abundant, rapid phagocytes.

• Macrophages / Monocytes – phagocytosis & antigen presentation.

• Dendritic cells – professional antigen-presenting cells (APCs).

• Natural Killer (NK) cells – destroy virally infected or tumour cells.

• Basophils & Mast cells – release histamine, start inflammation.

• Eosinophils – specialise against parasites (worms, some fungi).

Defensive Proteins

• Complement system – >30 plasma proteins that opsonise microbes, recruit phagocytes and form membrane attack complexes.

• Cytokines – soluble messengers (see dedicated section).

Four Key Processes (Internal Innate System)

1. Inflammation – local, rapid barrier & recruitment.

2. Fever – systemic temperature elevation.

3. Phagocytosis – ingestion & destruction of invaders.

4. Complement activation – opsonisation, lysis, amplification.

Inflammation Mechanics

• Trigger – tissue damage or pathogen presence → mast cells release histamine and cytokines.

• Cardinal signs

  • Redness (rubor): ↑ blood flow.

  • Heat (calor): ↑ blood volume & metabolism.

  • Swelling (tumor): plasma leakage (oedema).

  • Pain (dolor): nerve damage, chemical sensitisation, pressure.

  • Loss of function: sometimes included (functio laesa).

• Histamine actions → vasodilation + increased vascular permeability.

• Cytokines (e.g., IL-8) attract neutrophils.

Phagocytosis Step-by-Step

1. Chemotaxis – phagocyte migrates toward chemoattractants.

2. Adherence – binding via pattern-recognition receptors (PRRs). Opsonins (antibodies or complement $C3b$) greatly enhance this step.

3. Engulfment – pseudopodia surround particle → form phagosome.

4. Fusion – phagosome merges with lysosome → phagolysosome.

5. Digestion & killing – acidic pH, enzymes, reactive oxygen species.

6. Exocytosis – residual body expelled or antigens displayed on MHC → cell becomes an APC.

Cytokines at a Glance

Fever

• Cause – bacterial toxins or cytokines (e.g., IL-1) reset hypothalamic thermostat.

• Benefits – enhances interferon action, inhibits some microbes, accelerates repair.

Adaptive (Acquired) Immunity – Specific & Memory-Forming

Hallmarks

• Delayed onset (days) on first exposure; rapid on re-exposure.

• Specificity – recognizes unique antigens.

• Memory – basis of vaccination.

Key Cells

Antigen & Antibody Definitions

• Antigen – any molecule that elicits a lymphocyte response; commonly surface proteins on pathogens, but also toxins, pollen, etc.

• Antibody (Immunoglobulin) – Y-shaped protein from plasma cells; binds antigen → neutralisation, opsonisation, complement activation.

Cell-Mediated vs. Humoral Targets

• Cell-mediated (T cells) – best for intracellular pathogens: viruses, some bacteria, cancer cells.

• Humoral (B cells/antibodies) – best for extracellular pathogens: bacteria, toxins, free viruses.

Blood Clotting – Mechanistic Detail

• Enzyme: Thromboplastin initiates a cascade → prothrombin → thrombin → converts fibrinogen → fibrin.

• Outcome: fibrin mesh + platelets = thrombus sealing wound.

Practical / Clinical Examples & Scenarios

1. Bramble Scratch Case

   • Tissue injury → mast-cell histamine release.

   • Redness & heat: vasodilation.

   • Next-day pus: accumulation of dead neutrophils, bacteria & tissue debris = abscess; indicates ongoing phagocytosis.

2. HIV Transmission via Needle – inoculation route bypasses 1st line; virus enters bloodstream directly—illustrates importance of skin barrier.

3. Malaria Vector Control – reducing mosquito populations interrupts protist transmission—public-health application of transmission knowledge.

Summary Checklist (Exam-Style Revision)

1. Immunological vs. Non-Immunological Aims

   • Prevent entry vs. remove invader.

2. Physical Barriers (≥2) – skin, mucociliary escalator.

3. Chemical Barriers (≥2) – stomach acid, lysozyme in tears.

4. Internal Innate Processes (4) – inflammation, fever, phagocytosis, complement.

5. Phagocytosis Definition – engulfment & enzymatic digestion of particles by cells (steps listed earlier).

6. Cardinal Signs of Inflammation – redness, heat, swelling, pain, loss of function.

7. Two B-Cell Types – plasma (antibody factory) & memory (long-term immunity).

8. Cell-Mediated Targets – intracellular pathogens, cancer cells.

9. Body Adaptations Preventing Pathogen Entry – full list under first-line section.

10. Role of Cytokines & Opsonins – cytokines attract/activate phagocytes; opsonins coat microbes → enhanced adherence & ingestion.

Ethical / Practical Considerations

• Antibiotic stewardship – Gram status guides therapy to prevent resistance (e.g., MRSA).

• Vaccination strategies – rely on adaptive memory; herd immunity alters transmission factors.

• Autoimmunity & Tolerance – failure to distinguish self → chronic diseases; emphasises importance of regulatory pathways.