Microbial Immune Evasion Mechanisms Flashcards
What does balanced pathogenicity mean
The balance between the pathogen and the host in an infection
Properties of the microbe:
- Adhesins
- Toxins
- Capsule
Properties of the host:
- Natural barriers
- Defensive cells
- Antibacterial peptides
- Innate and adaptive immune response
What are virulence factors
- Promote colonisation and adhesion
- Evade host defence
- Promote tissue damage
Immune response against these may protect from disease
What aspects of immunity have pathogens overcome
- Natural defences - mucosal layers, skin
- Innate immunity - complement system and macrophages
- Adaptive immunity - antigen-specific and memory antibodies, T cells
Describe the classical pathway of the complement system
- Antibody binds antigen and molecule C1q forms a complex with it
- Triggers a set of cascade with C1, C4 and C2
- Activate of C3 convertase to form C3b
- Activation of C5 converts to form membrane attack complex
- Forms holes on the membrane of pathogen
Describe the lectin pathway of the complement system
- Lectin binds mannose on pathogens
- Triggers a set of cascade with MASPs, C4 and C2
- Follows the same C3 converts pathway as the classical pathway
Describe the alternative pathway of the complement system
- Pathogens and injured tissue are coated with elements of complement C3b
- Follows the same C3 converts pathway
- Leads to lysis or enhanced phagocytosis
What elements do bacteria have to prevent complement pathways activation
Bacterial capsules:
-Block binding of C3b
- Coating of non-fixing IgA
- Prevents C3b receptor access
- Factor H sequestration
- C5a proteases secretion
- Blebbing
what mechanism do bacteria have to prevent phagocytosis or survive it
- Leucocidins kill macrophages
- Protein A binds to Fc portion of IgG
- Blocking contact t with capsules
- CR3 receptors for safe route phagocytosis
- Prepare host cell for invasion via type 3 secretory systems - stops killing pathogen
- Inhibit phagosome to lysosome fusion
- Escape phagosome-lysosome to cytoplasm
- resist oxidative killing by catalase/peroxidase production
Describe how bacteria cause antibody inhibition
- Production of Protein A or Fc receptor
- Antibody doesn’t bind the bacteria but only the receptor
- Stops opsonisation and elimination
How do viruses survive inside the cell
- Hide from adaptive immunity by hiding in privileged sites
- Blocks MHC antigen presentation
- Surface uptake of host molecules
- Induce immunosuppression
- Antigenic variation
- Persistence, latency and reactivation
What disease can Streptococcus pneumonia cause
- Pneumonia
- Otitis
- Meningitis
Describe the pathogenic mechanism of streptococcus pneumoniae
- If strep colonises the nasopharynx they can stay there via adhesions and avoid death by sIgA proteases
- Inhalation into lungs via by-passing defence mechanisms such as surfactants
- In the lungs they can cause inflammation, and damage to endothelial cells and still avoid phagocytosis
- Causes pneumonia and lung damage
- Bacteraemia can cause meningitis and septicaemia
What elements do viruses posses to evade the immune system
- Latency - stay dormant in a privileged cell
- Decreased antigenic presentation
- Decreased MHC expression
- Mutation of epitopes
What does phenotype changes mean in antigenic variation
Changes in:
- colony morphology
- virulence
- serotype
- loose flagella
- change surface sugars
What does antigenic diversity/polymorphism mean
Genetically stable but alternative forms of antigens in a population of microbes
