3. Innate Immunity - 1st Line Defences

Question 1

Which 3 main factors determine the outcome of the host-pathogen relationship?

Answer 1

1. Host’s immune response
2. Pathogen’s infectivity (ability to infect/adhere to host)
3. Pathogen’s virulence (ability to damage host)

Question 2

Define the term ‘immune system’.

Answer 2

Cells and organs that contribute to immune defences against infectious and non-infectious conditions - recognise self vs non-self.

Question 3

Define the term ‘infectious disease’.

Answer 3

When the pathogen succeeds in evading and/or overwhelming the host’s immune defences.

Question 4

State 4 differences between the innate and adaptive immune responses.

Answer 4

Innate:

1. fast (seconds)
2. non-specific
3. no memory
4. no change in intensity

Adaptive:

1. slow (days)
2. specific (differentiates between different types of pathogens)
3. immunologic memory
4. changes in intensity (eg faster after re-infection due to memory)

Question 5

Which 4 innate barriers (1st line defences) prevent entry and limit growth of pathogens?

Answer 5

1. physical barriers
2. physiological barriers
3. chemical barriers
4. biological barriers

Question 6

Give 3 examples of innate physical barriers against infection.

Answer 6

1. skin
2. mucous membranes (mouth, respiratory tract, GI tract, genito-urinary tract) - have own lymphoid tissues to stimulate local immune response
3. bronchial cilia - expel mucous-bound pathogens

Question 7

Give 4 examples of innate physiological barriers against infection.

Answer 7

1. diarrhoea (eg food poisoning)
2. vomiting (eg food poisoning, hepatitis, meningitis)
3. coughing (eg pneumonia)
4. sneezing (eg sinusitis)

Not necessarily associated with relevant site of infection (eg pneumonia can also be associated with vomiting)

Question 8

Give 2 examples of innate chemical barriers against infection.

Answer 8

1. low pH (contributed to by presence of normal microflora esp. lactobacillus)
   - skin (5.5)
   - stomach (1.0-3.0)
   - vagina (4.4)
2. antimicrobial molecules
   - IgA (tears, saliva, mucous membrane) - prevents MO attachment
   - lysozyme (sebum, perspiration, urine) - hydrolyse bacterial peptidglycan wall
   - mucous (mucous membranes) - traps MOs to facilitate elimination
   - beta-defensins (epithelium) - form pores in bacterial wall
   - gastric acid and pepsin - inhibit bacterial growth and protease

Question 9

Explain how innate biological barriers prevent infection.

Answer 9

Involves the normal microflora = non-pathogenic microbes present in strategic locations (nasopharynx, mouth, skin, GI tract, vagina (lactobacillus spp) but absent in internal organs/tissues.

1. Compete with pathogens for attachment sites and resources
2. Produce antimicrobial chemicals
3. Synthesise vitamins (K, B12, other B vitamins)

Question 10

Under which conditions can the normal microflora become pathogenic? Why might this occur?

Answer 10

1. Normal flora is displaced from its normal location to a sterile location
   i) breaching skin integrity - skin bacteria in blood stream (eg skin loss (burns), surgery, injection drug users, IV lines (esp central line))
   ii) faecal-oral route - foodborne infection
   iii) faecal-perineal-urethral route (eg UTI in women)
   iv) poor dental hygiene/dental work (common cause of harmless bacteraemia)
2. Normal flora overgrows and becomes pathogenic in immuno-compromised host (eg diabetes, AIDS, malignant diseases, chemotherapy)

Question 11

Which patients are at high risk of serious infection due to normal flora displacement? What do these patients require?

Answer 11

1. asplenic (and hyposlenic) patients
2. patients with damaged or prosthetic valves
3. patients with previous infective endocarditis

Antibiotic prophylaxis

Question 12

Why might the normal microflora become depleted? Give examples of diseases this could cause.

Answer 12

Depleted by antibiotics.

• intestine… severe colitis (Clostridium difficile)
• vagina… thrush (Candida albicans)

Question 13

Which innate immune defences are present in the respiratory system to prevent respiratory infections developing?

Answer 13

1. ciliated epithelium - moves particles towards pharynx and down digestive tract
2. goblet cells secrete mucus - physical barrier to infections, and contains lysozymes and IgA
3. normal respiratory flora - competition
4. cough reflex - aids expulsion of harmful substances
5. rich vasculature of resp. system - high levels of neutrophils and alveolar macrophages