Pulmonary Defence Mechanisms

• What are the main categories of particles that can be inhaled into the respiratory system?

  1. Biological particles 
  2. Particulate matter (PM) 
  3. Chemical irritants 
  4. Organic dusts 
  5. Inorganic dusts 
• What is particulate matter (PM), and how is it classified based on size?

  • PM10 (≤10 µm): Can reach the bronchi and upper airways.
  • PM2.5 (≤2.5 µm): Can penetrate deep into the alveoli.
  • Ultrafine particles (<0.1 µm): Can cross into the bloodstream.
• What are the main sources of particulate matter (PM) in the air?

  • Natural sources: Wildfires, volcanic ash, dust storms, sea spray
  • Anthropogenic sources: Vehicle emissions, industrial pollution, tobacco smoke, burning fossil fuels
• What are the major types of airborne pathogens that can cause respiratory infections?

  1. Bacteria (e.g., Mycobacterium tuberculosis, Streptococcus pneumoniae)
  2. Viruses (e.g., Influenza, SARS-CoV-2, Rhinovirus)
  3. Fungi (e.g., Aspergillus spp., Histoplasma capsulatum)
  4. Protozoa (e.g., Pneumocystis jirovecii in immunocompromised individuals)
• How do bacteria enter the respiratory system, and what diseases can they cause?

  • Transmission: Droplet inhalation, contaminated aerosols
  • Examples of diseases:
  • Streptococcus pneumoniae → Pneumonia
  • Mycobacterium tuberculosis → Tuberculosis
  • Bordetella pertussis → Whooping cough
• What are common airborne viruses that affect the respiratory system?

  • Influenza virus → Flu
  • Rhinovirus → Common cold
  • SARS-CoV-2 → COVID-19
  • Respiratory syncytial virus (RSV) → Bronchiolitis, pneumonia
•  How do fungi affect the respiratory system, and who is most at risk?

  • Common airborne fungal pathogens:
  • Aspergillus spp. → Aspergillosis
  • Histoplasma capsulatum → Histoplasmosis
  • Cryptococcus neoformans → Cryptococcosis
  • Risk factors:
  • Immunocompromised individuals (e.g., HIV/AIDS, chemotherapy patients)
  • Chronic lung diseases (e.g., COPD, cystic fibrosis)
• What is Pneumocystis jirovecii, and why is it a concern?

  • A fungal-like organism that causes Pneumocystis pneumonia (PCP)
  • Affects immunocompromised patients, especially those with HIV/AIDS
  • Transmission occurs via inhalation of airborne cysts
• Give examples of chemical irritants that can be inhaled into the respiratory system

  • Toxic gases
  • Fumes
  • Smoke
• Give examples of organic dusts that can be inhaled into the respiratory system

  • Pollen
  • Animal dander
  • Mould
• Give examples of inorganic dusts that can be inhaled into the respiratory system

  • Asbestos
  • Silica
  • Coal dust
• What are the primary physical barriers that protect the respiratory system from infection?

  1. Nasal hairs (Vibrissae) – Trap large airborne particles.
  2. Mucociliary escalator – Moves trapped particles upward for clearance.
  3. Cough reflex – Expels irritants and pathogens.
  4. Sneezing – Expels foreign particles from the nasal cavity.
  5. Tight junctions between epithelial cells – Prevent pathogen entry.
  6. Airway branching – Deposition of particles due to turbulence.
• What is the mucociliary escalator, and how does it function?

  1. Mucus production (Goblet cells & Submucosal glands) – Traps microbes and debris.
  2. Ciliated epithelial cells – Beat rhythmically to move mucus toward the pharynx for swallowing or expectoration.
• How does mucus contribute to respiratory defence?

  • Traps pathogens & particles before they reach the lower airways.
  • Contains antimicrobial substances like lysozymes, defensins, and IgA.
  • Hydration of airways – Prevents drying, which can increase infection risk.
• How does airway branching contribute to pulmonary defence?

  The bifurcation of airways causes turbulence, leading to deposition of inhaled particles onto mucosal surfaces, where they can be cleared.
• What is the role of the epiglottis in pulmonary defence?

  The epiglottis prevents aspiration by covering the trachea during swallowing, reducing the risk of infection from ingested materials.
• Which immune cells play a key role in pulmonary defence?

  1. Alveolar macrophages – Phagocytose pathogens and debris in alveoli.
  2. Neutrophils – Rapidly recruited to fight bacterial infections.
  3. Dendritic cells – Present antigens to initiate adaptive immunity.
  4. Mast cells – Release histamine, triggering inflammatory responses.
  5. Natural killer (NK) cells – Destroy infected and abnormal cells.
• What is the role of alveolar macrophages in respiratory defence?

  • Phagocytose inhaled pathogens & particulates.
  • Secrete cytokines (IL-1, IL-6, TNF-α) to recruit immune cells.
  • Suppress unnecessary inflammation to protect lung tissue.
• How do neutrophils contribute to respiratory immunity?

  • First-line response to bacterial infections.
  • Release proteolytic enzymes & reactive oxygen species (ROS) to kill microbes.
  • Undergo NETosis – release DNA to trap pathogens.
• What is the role of IgA in respiratory defence?

  • Secretory IgA (sIgA) is the main antibody in mucus.
  • Neutralises pathogens before they attach to epithelial cells.
  • Prevents colonisation by binding microbes and promoting clearance.
• What are surfactant proteins, and how do they aid immunity?

  • Produced by Type II alveolar cells.
  • SP-A & SP-D opsonise pathogens for phagocytosis.
  • Regulate inflammatory responses to avoid lung damage.
• How does the complement system enhance pulmonary defence?

  • Opsonisation – Tags microbes for easier phagocytosis.
  • Chemotaxis – Recruits immune cells.
  • Membrane Attack Complex (MAC) – Lyses bacterial cells.
• How do dendritic cells link innate and adaptive immunity in the lungs?

  • Sample inhaled antigens in the airways.
  • Migrate to lymph nodes to activate T-cells.
  • Initiate adaptive immune responses against pathogens.
• What is the role of T-cells in pulmonary immunity?

  • CD4+ (Helper T-cells) – Activate B-cells & macrophages.
  • CD8+ (Cytotoxic T-cells) – Kill virus-infected cells.
  • Regulatory T-cells – Prevent excessive inflammation.
• What is the role of CD4+?

  Activate B cells & macrophages