L20 - Microbial Immune Evasion Mechanism Flashcards
What is the purpose of virulence factors?
To evade host defences
promote colonisation and adhesion
promote tissue damage
to establish infection
What aspect of immunity have pathogens evolved
to overcome or avoid?
Natural defences - mucosal layers, skin,
Innate immunity
complement system
macrophages
Adaptive Immunity - antigen specific and memory
antibodies T cells – CD4 helper cells and CD8 cytotoxic
cells
How does bacteria stop complement?
Bacteria capsules have polysacharrides that prevent the triggering of the complement.
Soek bacteria have properties that allow for preferential binding of the wrong antibody. Example:
In mucosal surfaces, some bacteria can get coated with IgA instead of IgM, leading improper activation of classical pathway.
Some capsules blocks c3b binding, which is an opsonin part of the alternative pathway
Describe the different mechanisms that enable life inside macrophages?
- Direct phagocytosis via CR3 - without reactive oxygen species.
- Rearrange the actin cytoskeleton to involve them in a safe manner
- prepare the cell via the Type 3 secretory systems, so they are phagocytosed in a safe manner.
- Can resist digestion by producing proteins that block and neutralise components that are highly toxic and oxdising
- Have toxins that allow them to escape into cytoplasm before lysosome fusion
- Inhibit PL fusion, and stops acidification of the environment
- controls the antigen presentation
Describe how microbes cause antibody inhibition by FC receptors?
Some bacteria produce a protein that mimics the FC receptor which then sequesters the antibodies. Blocks the effective function of the antibody. Stops opsinisation
How is persistance an immune evasion strategy?
Microbes infect susceptibles
Microbe remains latent
Microbes reactivates and infects next generation of susceptibles
What are phenotype changes in terms of Antigenic Variation?
Phenotype changes - colony morphology, virulence, serotype
loose flagella, change surface sugars
What are Antigenic diversity/ polymorphisms in terms of Antigenic Variation?
Antigenic Diversity/ polymorphisms
- genetically stable and alternative forms of antigens
in a population of microbes
What is Antigenic Variation?
Antigenic Variation
- successive expression of alternative forms of an antigen
in a specific clone or its progeny
What is Phase Variation?
Phase variation - ON/OFF of an antigen at low frequency
- occurs - during course of infection in an individual host
- during spread of microbe through a community
What is Antigenic Drift?
Mutation and selection
What is Antigenic Shift?
Gene reassortment
How can Viruses evade the immune response?
MHC mimics –
block killing by NK cells
Block antigen
processing by TAP
Rapid growth and transmission
prior to adaptive immunity e.g. colds
Latency reactivation
Blockage of cell cycle
progression
Dysregulation
of apoptosis
Hide inside cells
(ANTIGENIC DRIFT AND SHIFT)
How can Bacteria evade the immune response?
Prevents opsonin binding
Inhibit complement
activation
Survival inside
macrophages
Block uptake by
macrophages
