microbial immune evasion mechanisms

Question 1

how may pathogens resist complement

Answer 1

LPS layer and capsule prevent complement from binding = failure to start cascade.
some bacteria coat themselves with IgA (which doesnt fix complement) = failure to start cascade

Question 2

what is phagocytosis

Answer 2

primary mechanism by which pathogens are dealt with. generally done via opsonisation.
engulf bacteria

Question 3

how may bacteria combat phagocytosis

Answer 3

producing leucocidins (kill leukocytes)
express protein =binds to Fc of IgG .'. prevent opsonisation 
block contact of opsonins via teh capsule

Question 4

how do intracellular pathogens (eg bacteria living in macrophages) evade the immune system

Answer 4

inside macrophage .’. immune system cant reach
promote own uptake
prevent phagolysosome fusion: .’. not killed
escape phagolysosome .’. get into cytoplasm
produce catalses/perioxidase .’.resists oxidative kiling

Question 5

how may microbes evade the adaptive immune system

Answer 5

Production of Fc Receptors by Microbes
Concealment of Antigen:
Immunosuppression
Antigenic variation 
Persistance/latency/reactivation

Question 6

how does microbial Fc receptors allow for evasion of the adaptive immune system

Answer 6

certain pathogens have genes -> express Fc on their own surface .’. microbe can bind antibodies the wrong way around .’. antibody cannot opsonise properly

Question 7

how does microbial concealment of antigen allow for evasion of the adaptive immune system

Answer 7

microbe conceals itself .’. not recognised by immune system .’. microbe can divide etc
does this by: hidiing inside cells/in privledged sites, `blocking antigen MHC presentation

Question 8

how does immunosuppression by the microbe allow for evasion of the adaptive immune system

Answer 8

intracellular organsms inhibit MHC-antigen expression
lots of intracellular bacteria and viruses can decrease receptors on surface of cells or induce/prevent apoptosis
can alter TH1/2 balance of the immune system

Question 9

how does antigenic variation of the microbe allow for evasion of the adaptive immune system

Answer 9

overcomes adaptive immunity by having different antigens that allow multiple strains to exist in a population and continue infection

Question 10

what is streptococci Pneumonia

Answer 10

causes pneumonia and is a gram +ve bacterium

Question 11

pathology of streptococcus pneumoniae

Answer 11

transmites via aerosol. enters nasopharynx and start to colonise. here.
it can cause middle ear infections. can be inhaled into lungs

Question 12

how does S. Pneumoniae evade immune mechanisms of the nasopharynx

Answer 12

releases adhesins that bind to cell surface molecules and release sIgA proteases to break sIgA that is present in the mucous

Question 13

how does S. Pneumoniae evade immune mechanisms of the lungs

Answer 13

lung has tight epithelial junctions, surfactant and some antimicrobial peptides
bacteria release sIgA proteases. also releases Pneuomolysin (toxin) causing lysis of epithelial surface of the lungs

Question 14

immune defences in the lung

Answer 14

tight epithelial junctions, surfactants (acts like mucus) and some anti-microbial peptides.

Question 15

how does S. Pneumoniae evade the adaptive immune response

Answer 15

hard for phagocytosis due to capsule surrounding it
Have teichoic acids on their surface = inflammation damaging the lung
coninuing damage to lung -> pneumonia and see dead neutrophils, pus, dead tissue accumulate. all this damage means bacteria can access the blood .’. bacteraemia

Question 16

how does S. Pneumoniae evade the adaptive immune response

Answer 16

hard for phagocytosis due to capsule surrounding it
Have teichoic acids on their surface = inflammation damaging the lung
coninuing damage to lung -> pneumonia and see dead neutrophils, pus, dead tissue accumulate. all this damage means bacteria can access the blood .’. bacteraemia

Question 17

what kind of immunity is needed to combat viruses

Answer 17

are intracellular pathogens so the cell-mediated immunity is required to combat them.

Question 18

how do viruses evade the immune system

Answer 18

latency; decrease antigenic presentation whilst inside cells; decrease MHC expression; MMC mimics

Question 19

what is the immune strategy - persistence

Answer 19

it will lay latent and then when it reactivates it can be shed and infect new naive individuals.

Question 20

how do viruses decrease antigenic presentation whilst inside cells

Answer 20

eg herpes simplex prevents peptides being transferred by MHC by binding and inhibiting TAP

Question 21

how do viruses decrease MHC expression

Answer 21

Cytomegalovirus (CMV) can do this, CD8+ cells don’t know that the cell is infected

Question 22

how do viruses evade the immune system - MHC mimics

Answer 22

produces molecules that mimic the normal MHC

Question 23

How do MHC 1 work

Answer 23

all nucleated cells express MHC1, is scanned by CD8+. If MHC1 = self-peptide nothing happens. If infected with virus then MHC will hold viral peptides, recognised by CD8+ .’. becomes CTL and kills infected cell

Question 24

what is an NK cell

Answer 24

natural killer cell work by generically detecting MHC on cell surface (which tells that cell is healthy

Question 25

how do CD8 and NK cells work together to determine whether

Answer 25

some viruses downregulate MHC1 to avoid recognition by adaptive immune system (bc if recognised all viruses inside cell are killed)
but if downregulates MHC1 too much, NK cell will scan it and realise MHC is missing indicating cell is not healthy .’. kill it

Question 26

how may viruses evade CD8+ and NK cell

Answer 26

some viruses eg CMV contain a gene that expresses an MHC homologue that acts like the normal cellular MHC. Viral peptides are expressed on decoy MHC; CD8 unable to recognise .’. unable to kill it. MHC expressed on cell surface and as it is self antigen NK cells do not kill it

Question 27

how may viruses evade the immune system by mutating

Answer 27

such rapid mutation that their epitopes can mutate .’. even same individual may contain different mutants .’. can escape CD8 and B cells

Question 28

what is antigenic diversity/polymorphism

Answer 28

in a population of hosts there are many different forms of antigens .’. species so diverse that it can survive well in the population

Question 29

what is antigenic variation

Answer 29

individual organism can change its surface antigens by altering antigen genes that it can switch on and off. once antibodies have been made to one antigen it switches off that expression and switches on another antigen presentation

Question 30

what is phase variation

Answer 30

a mechanism of antigenic variation - some pathogens have ability to recombine its genetic material in order to code for new antigen variants

Question 31

what is gonorrhoea

Answer 31

a gram -ve coccus that causes urethritis (inflammatory and pyrogenic infection of anterior urethra)

Question 32

how does gonorrhoea evade the adaptive immune response

Answer 32

undergoes phase variation

interacts with host cells but then varies its components

Question 33

why can a person be infected with influenza again and again

Answer 33

has a segmented ssRNA (8 segments) allows it to recombine chunks of its genome
has about 15 types of HA and 9 types of NA gives lots of potential combinations in the population
undergoes rapid point mutation bc lack proof reading ability
.’. changes presentation

Question 34

what is antigenic drift

Answer 34

natural point mutations occur due to viral replication.

accumulation of mutations within the genes that code for antibody-binding sites lead to new surface antigens

Question 35

what is antigenic shift

Answer 35

2+ different strains of virus combine =new subtype having a mixture of the surface antigens of original strains.
.’. new antigens that no one has immunity to