2: Immune Evasion

Question 1

How are bacteria detected by the immune system

Answer 1

Complement system - group of proteins that opsonize or lyse bacteria
Antibodies - proteins that can opsonize bacteria
Phagocytes - immune cells that detect and kill opsonized bacteria

Question 2

Key steps of complement cascade

Answer 2

Initiation
Generation of C3 convertase - bacterial killing by phagocytosis
Generation of C5 convertase
MAC formation - killing by lysis

Question 3

3 complement pathways

Answer 3

classical pathway
lectin pathway
alternative pathway

Question 4

Classical pathway

Answer 4

IgG and IgM bind to antibody - trigger
C4b and C2a form C3 convertase
Cascade reaction forming MAC

Question 5

Lectin pathway

Answer 5

lectins (MBL and CRP) binding to pathogen - trigger
C4b and C2a form C3 convertase
Cascade reaction causing MAC

Question 6

Alternative pathway

Answer 6

polysaccharide on bacteria surface triggers alternative pathway
C3b and Bb form C3 convertase
Cascade reaction forms MAC

Question 7

Membrane Attack Complex (MAC)

Answer 7

result of each of the complement pathways
creates pores in pathogen membranes -> influx of fluid, cell lysis and destruction of pathogen

Question 8

Antibody Opsonisation

Answer 8

Antibody binds to antigen
1. makes bacteria more recognisable
2. easier to engulf bacteria
triggers classical pathway of complement system

Question 9

Phagocytes

Answer 9

engulf pathogen, contain it in phagosome and digest it using enzymes

C3b and IgG act as opsonins (tags) to mark foreign pathogen for phagocytosis
makes process more efficient

Question 10

Examples of phagocytic cells

Answer 10

Neutrophil
Monocyte
Macrophage
Dendritic cell

Question 11

Innate immunity

Answer 11

rapid, provides broad protection
a deficiency in complement system, lack of antibodies or phagocytes lead to impaired innate immune system
leads to an opportunity to cause infections

Question 12

Immune evasion

Answer 12

How pathogens survive killing by immune response so they can continue to infect the host

Question 13

Mechanisms of immune evasion

Answer 13

Avoid recognition
Inhibit phagocytosis
Kill immune cells
[ survive inside cells
disrupt immune signalling
mimic host molecules ]

Question 14

Staphylococcus aureus

Answer 14

Gram +
Commensal in skin and mucous membranes
Opportunistic - can cause disease as MRSA ;
Skin and soft tissue infections
Osteomyelitis
Endocarditis
Bacteraemia and Sepsis

Question 15

How does S. Aureus evade the complement system

Answer 15

Aur protease degrades C3 –> prevents C3b deposition, C3a formation

Sbi protein binding to IgG, preventing recognition by C1 –> prevents formation of C3 convertase of classical pathway

SCIN protein binds to C3bBb –> prevents formation of C3 convertase and C5 convertase

SSL7 protein binds to C5 –> prevents C5 convertase activity on C5

Question 16

How does S. Aureus evade antibody response

Answer 16

bacteria express capsule on surface = hides antigenic structures that could be detected

Protein A bind antibodies via their Fc region not Fab region = prevents normal opsonisation

Proteases cleave or modify antibodies = prevents normal opsonisation
( CAN converts plasminogen into plasmin OR break down antibody)

Question 17

What phagocytes does Alpha-toxin produced by S. Aureus kill

Answer 17

neutrophil
monocyte
macrophage

Question 18

What phagocytes does PVL produced by S. Aureus kill

Answer 18

neutrophil
monocyte
macrophage

Question 19

What phagocytes does LukAB produced by S. Aureus kill

Answer 19

neutrophil

Question 20

What phagocyte does PSMs produced by S. Aureus kill

Answer 20

neutrophil
monocyte
macrophage
dendritic cell

Question 21

Viral Immune Evasion

Answer 21

Virally infected cells release IFNs (interferons) which initiate antiviral state -> block replication
IFNs also activate NK cells
Many viruses escape antibody recognition by blocking IFN production

Question 22

Types of interferons

Answer 22

Type I IFNs are IFN-a and IFN-B
Type II IFN is IFN-y ; produced by activated T-cells and NK cells
Type III IFN is IFN-λ

Question 23

Natural Killer cells are activated by

Answer 23

IFN-a and IL-12

-kills virus-infected cells

Question 24

Macrophages inactivate

Answer 24

opsonised virus’ , filter viral particles from blood

Question 25

Dendritic cells initiate

Answer 25

CD4 and CD8 response

Question 26

CD4 T-Cells respond to

Answer 26

MHC 2

Question 27

CD8 T-Cells respond to

Answer 27

MHC 1

Question 28

Neutrophil functions at site of inflammation

Answer 28

Pathogen recognition
pathogen clearance
cytokine secretion