Intro to resp infections Flashcards
explain the barriers to infection
1.Mucus and cilia.
Muco-ciliary escalator helps transport micro-organism-laden mucus up into the pharynx where it can be coughed up or swallowed.
Mucus contains microbicidal factors such as lysozyme, as well as secretory IgA that binds specific micro
organisms to mucin
Explain normal flora as a barrier to infection
Presence of healthy normal flora (harmless commensal micro-organisms)
* occupies niches, competes for nutrients, secretes antimicrobial substances
Explain innate immune system as a barrier to infection
Soluble factors: Lysozyme, complement, acute phase proteins, e.g. C-reactive protein, interferon,
other cytokines
* Cells:
* Phagocytes (macrophages especially alveolar macrophages, and neutrophils)
* Innate lymphoid cells including natural killer cells
How do respiratory pathogens overcome host defenses? first step?
They attach to respiratory epithelium
* Microbes may have ‘adhesins’ that bind to host ‘receptors’
* Called a receptor, but that isn’t their own purpose and function, just hijacked by microbe
* Presence of suitable receptors determines microbial tropism
* e.g. ‘avian’ influenza can’t usually infect humans because our respiratory epithelium lacks the avian
type receptors that virus needs to attach
step 2 of overcoming host defenses
They inhibit ciliary activity
* Viruses (various ones)
* Bacteria: Pertussis, Pseudomonas aeruginosa, Haemophilus influenzae (usually via toxins produced by
bacteria)
What other factors inhibit cilia?
NB Environmental factors like atmospheric pollution, cigarette smoking, very dry air can also inhibit ciliary
function
People exposed to these factors or those with chronic abnormalities affecting cilia e.g. cystic fibrosis or
chronic lung disease are already at increased risk of respiratory infections
step 4 of overcoming host defenses
They avoid killing by alveolar macrophages
* M. tuberculosis, Legionella
* Note: Macrophages may be disabled by prior exposure to asbestos and other dusts. Smoking
also decreases activity of macrophages. These environmental factors predispose people to
respiratory infections
List the requirements of professional invaders. (4)
Adhesion to normal mucosa (in spite of mucociliary system)
Ability to interfere with cilia
Ability to resist destruction in alveolar macrophage
Ability to damage local (mucosal,
submucosal) tissues
List the requirements of secondary invaders
Initial infection and damage by
respiratory virus (e.g. influenza virus)
Local defences impaired (e.g. cystic fibrosis)
Chronic bronchitis, local foreign body or tumour
Depressed immune responses (e.g. AIDS, neoplastic disease)
Depressed resistance (e.g. elderly, alcoholism, renal or hepatic disease)
Difference between surface pathogens and systemic pathogens
Surface pathogens multiply and infect the surface, while systemic pathogens penetrate to cause systemic infections
Characteristics of surface pathogens
Most of the respiratory pathogens are surface pathogens
* Characterised by rapid multiplication and spread over cell surfaces
* Local mucosal defences most important in preventing infection
* Typically short incubation times
* Typically rely mainly on rapid-acting innate immune system for control of infection
* acquired immune system not usually quick enough to assist, unless primed for more rapid
secondary response via previous infection or vaccination
There are a few exceptions where pathogen invades via respiratory tract but causes systemic infection. List them (4)
Mycobacterium tuberculosis
* Legionella
* EBV
* Measles
Who is at risk of respiratory infections?
Age
* Very young
* (Very) old
* Impaired immune status
* HIV
* Splenectomy
* Alcohol
* others
* Preceding viral respiratory infection
* Underlying lung diseases
* Cystic fibrosis
* Bronchiectasis
* Cigarette smoking and pollution
* Those with increased risk of exposure to pathogens
