set 2.4-2.7

Question 1

what are the two strategies viruses use to avoid host defences?

Answer 1

Passive evasion:
-antigenic drift and shift
-internal sanctuaries
-immune tolerance
Active evasion:
-immunosuppression
-blockage of MHV antigen presentation

Question 2

what is antigenic drift?

Answer 2

• mostly happens from mutations
• changing epitopes on antigens (mutations produce number of serotype variants)
• often occurs during geographical confinement
• mostly seen with RNA viruses

Question 3

what are the advantages and limitations of antigenic drift?

Answer 3

• host can develop immunity, no vaccine can be produced
• potential unlimited access to host
• can only change so much without risk (counteracted by antigenic conservatism)

Question 4

why is antigenic drift more common in RNA viruses?

Answer 4

-viral RNA replicase and reverse transcriptase don’t do editing so mistakes can be made easily (some mutations affect antigenic properties of viral surface proteins)

Question 5

what occurs in HIV due to the high levels of antigenic drift?

Answer 5

• antibodies produced in early stages of infection are useless to neutralize newer generations of virus created within the host during the infection cycle
• HIV does this

Question 6

what is antigenic shift?

Answer 6

an abrupt, major change in characteristics of viral surface proteins

• segmented viruses can undergo assortment of genes during antigenic shift and express new surface antigens
• two strains of a virus undergo reassortment to form new strain

Question 7

why is antigenic shift advantageous to viruses?

Answer 7

-new reassorted virus expresses a new surface antigen that’s not recognized by host antibodies

Question 8

how do internal sanctuaries work for some viruses to avoid detection by host immune system?

Answer 8

• viruses hide in cells with lower immune surveillance

ex. HSV hides in non dividing sensory neurons

Question 9

what is immune tolerance?

Answer 9

• targets fetuses and neonates

- have immature immune systems (leads to tolerance and surpasses specific immune response to virus)

Question 10

why are viruses that rely on immune tolerance limited?

Answer 10

-less hosts since they have to infect babies

Question 11

what is immunosuppression?

Answer 11

• suppression of one or more immune responses

- can be transient or permanent

Question 12

how does immunosuppression work?

Answer 12

• induces regulatory T cells (used to inhibit immune response)
• inhibiting T cell proliferation
• host can’t use immune responses against virus anymore

Question 13

how do some viruses interfere with the hosts complement?

Answer 13

• interact with complement regulatory proteins to block complement activation and neutralization of virus particles
• could interfere with chemokines, cytokines, antibody-mediated viral clearance, NK and dendritic cells, antigen processing and presentation, and prevent apoptosis
• useful for virus because complement can’t be activated and virus particles won’t be neutralized
• ex. herpesvirus mimics complement regulatory proteins to block complement