Pulmonary defence mechanisms Flashcards
why are the lungs a site of immunological invulnerability
- Fast movement of air in and out of the lungs- place limits on level of filtering/barrier structures possible
- Efficient gas exchange - large S.A, thin membrane, warm, moist environment at blood gas interface - microbes can thrive
Potentially damaging particles/organisms
- Microorganisms – bacteria, viruses, fungi, helminths
- Allergens – dust, pollen
- Organic particles – occupational exposures, pollution (e.g. diesel particulate matter)
- Toxic gases – carbon monoxide, sulphur dioxide, nitrogen dioxide
What size/types of particles can air contain
Large particulates
Foreign body aspiration (e.g. food, liquid, choke hazards)
Fine particulate material
- Pollution (<2.5 - 1000 μm)
- Dust (0.1-1000 μm)
- Pollen (10 - 100 μm)
Microscopic pathogens
- Fungal spores (2 - 10 μm)
- Bacteria (0.5 - 5 μm)
- Viruses (< 1 μm)
Physical features to limit risk of infections
Large scale = nasal hairs, nasal turbinates, branching airway structure
Micro scale = cilia, mucus
Protective reflexes to limit risk of infection
Coughing, sneezing, expiratory reflex
Immunological defence system to limit risk of infection
Lung resident immune cells (e.g. alveolar macrophages)
Structural cells (epithelial cells)/innate immunity
Antimicrobial proteins
Nasal hairs
Filter out large particles (>10μm) present in the air.
Nasal turbinates/conchae
Are mucous membrane lined, ridged structures within the nasal cavity that help to warm and humidify air before it reaches the airways/lungs as well as help to filter out particles larger than ≈ 2μm
Make up of mucus
Consists of a gel with elastic and viscous properties, which consists of 97% water and 3% solids (mucin, other proteins, salts, lipids), and also contains lysozyme and various antimicrobial proteins to destroy trapped microorganisms.
Mucus layer function
The mucus gel layer lies on top of a periciliary layer (≈ 7μm deep) which provides a media of low viscosity in which cilia can beat. Maintaining sufficient periciliary layer depth is critical to effective mucociliary clearance for this reason. As the respiratory tract is lined by ciliated epithelium
Cilia
The coordinated beating of cilia produces a wave of movement that propels the mucus gel layer towards the pharynx, where it is then swallowed or expelled.
How is cilia beating produced
Cilia beating and mucociliary clearance is produced by rhythmic movement of individual cilia
- Each individual cilia moves backward and forwards, the cilia only contacts the mucus gel layer during the forward stroke, as the cilia bends during the reverse stroke so that its tip passes beneath the mucus layer. Thus, the gel layer is propelled in one direction.
Cystic fibrosis & chronic bronchitis
mucus clearance is impaired leading to recurrent respiratory infections and resulting inflammation/tissue damage.
3 most distinct reflexes to remove harmful particles
sneezing
coughing
laryngeal expiratory reflex
What activates the 3 distinct reflexes
Nociceptors within different parts of the upper respiratory tract
Afferent sensory neurons
Transmit impulse to breathing centres within the brain
Efferent signals
- Then transmitted to specific respiratory muscles, glottis and airways to initiate a coordinated respiratory effort which rapidly expels air from nasal cavity/airways/lungs
- Via parasympathetic & motor nerves
How is sneezing initiated
- Stimulation of sensory receptors within nasal cavity
- Deep inspiration phase
- Compression phase (glottis is closed leading to pressure build up)
- Final expiration phase which air is expelled
How is coughing initiated
Stimulated by receptors within the larynx and large airways
Can also be voluntary
Involves bronchoconstriction to further increase expulsion pressure
Laryngeal reflex
Short, forcible expiratory effort without a preceding inspiration
How is a laryngeal reflex stimulated
Stimulation of sensory receptors within the vocal folds
Role of laryngeal reflex
Prevent foreign bodies entering airways and to expel phlegm in upper respiratory tract
How does the branching structure of airways help to filter particles
- Increase filtering of air and prevent particles form reaching lower respiratory structures
- When inhaled air reaches and airway branching site airflow changes from laminar to semi-turbulent flow pattern, increasing particle deposition - particles come into contact with mucus lined airway wall
Alveolar macrophages
Resident phagocytes within the lung and develop from progenitors produced in the bone marrow that migrate to the lung. AM are found within the airspace (and can travel between alveoli) and phagocytose pathogens, foreign material and cell debris, digesting it into residual material that is subsequently removed by the lymphatic system.
Important role of alveolar macrophages
Prevent respiratory infections
How does airway/lung microbiota help to maintain immunological balance
The mucosal surfaces of the respiratory system are not sterile (even in healthy individuals) – they are colonised by huge numbers of commensal bacteria.
These organisms play important roles in resisting infection by other pathogens and the development/modulation of a healthy immune system.
