Session 2 ILOs - Infection Model and Innate Immunity

Question 1

Describe a model of infection

Answer 1

The infection model acts as a scaffold to outline all aspects of an infection:

• The pathogen that causes it
• The symptoms the patient may present with
• The way in which the patient acquired the infection
• Why/how the pathogen causes the symptoms
• The management of the patient
• Likely prognosis/outcome for the patient

Question 2

Describe the human microbiome

Answer 2

The human microbiome is the total number and diversity of microbes found in and on the human body

Question 3

Identify microbes commonly found on the skin and other body sites

Answer 3

Skin:
(gram +ve cocci)
- Staphylococcus aureus
- Staphylococcus epidermidis
- Streptococcus species
(gram +ve bacilli)
- Corynebacterium species
- Propionibacterium acnes
 Mouth:
- Streptococcus mutans (gram +ve bacteria)

GI tract:
- Escherichia coli or E.Coli (gram -ve bacteria)

Upper respiratory tract:
- Streptococcus pneumonia (gram +ve bacteria)

Vagina:
- Candida albicans (yeast)

Question 4

Describe how microbes, normally present as commensals (part of the microbiome) can cause disease / infections in certain situations and give examples

Answer 4

If microbes can breach the innate barrier, e.g. skin or the mucosal membranes, then they can become infections where they can damage host tissue

Example:
- Bacteria normally present on the skin that breach the skin barrier either through cracked skin or cuts, can enter into the dermis and subcutaneous tissue leading to cellulitis

Example:
- Commensal Fungi (yeast) in the vagina e.g. candida albicans is kept in check by lactobacillus aureus. However if the level of lactobacillus aureus is reduced e.g. through antibiotics, then the candida albicans can grow leading to thrush

Question 5

Describe the different innate barriers to infection, including physical, physiological, chemical and biological and how they work to prevent microbe ingress

Answer 5

(Physical)
Skin:
- Tight junctions between keratinocytes prevent the ingress of microbes/pathogens present on the skin

Mucous membranes:
- Mucus produced by these membranes help to trap

(Physiological)
Diarrhoea:
- From food poisoning, aims to get the microbe out of the body

(Chemical)
Tears, saliva etc:
- Contains IgA which binds to the microbe and prevents it from binding tot he mucosal membrane

Stomach acid:
- Very low pH environment that is hostile to microorganisms

(Biological)
Normal flora
- Present at strategic locations to entry, however not present in internal organs or tissues (this is pathogen if there is!)

Question 6

Describe the second line of defence, including the role of phagocytes and chemicals (complement and cytokines)

Answer 6

Phagocytes:
- Neutrophils and macrophages ingest particles with the help of opsonins which enhance their phagocytic ability

Complement:

• These are a system of plasma proteins that collaborate to protect against bacterial infection which is activated by antigen-antibody binding or binding with cell wall components
• In response, it can result in chemotaxis, activation of phagocytes and cause vasodilation and opsonisation

Question 7

Describe how pathogens are recognised by cells of the innate immune system

Answer 7

Epithelial and endothelial cells express pathogen recognition receptors (PRRs) on their surface that recognize pathogen-associated molecular patterns (PAMPs) from microorganisms

In addition, coating proteins called opsonins bind to microbial surfaces and leads to enhanced attachment of phagocytes

Question 8

Name and describe important opsonins

Answer 8

Complement proteins:

• C3b
• C4b

Antibodies

• IgG
• IgM

Acute phase proteins:

• C-reactive protein (CRP)
• Mannose-binding lectin (MBL)

These are essential for clearing encapsulated bacteria (these can only be cleared by opsonins)