14. Despite humans having a highly evolved immune system, pathogens are able to evade
the immune system using various strategies. Which of the following immune evasion
strategies cannot be employed by a virus?
a. Antigenic shift
b. Antigenic switching
c. Antigenic drift
d. All of the above

Viruses employ antigenic shift, drift, and other mechanisms to evade the human immune system despite its sophistication. The correct answer to the question is c. Antigenic drift, as this strategy is primarily associated with gradual mutations in viruses, but all options represent viable viral tactics; however, in standard immunology contexts for CSIR NET, antigenic switching is distinguished as a non-viral mechanism.​

Option Breakdown

• a. Antigenic shift: Viruses like influenza A undergo major genetic reassortment, creating novel subtypes that bypass existing antibodies and cause pandemics.​

• b. Antigenic switching: Protozoan parasites such as Trypanosoma brucei switch variant surface glycoproteins (VSGs) via gene recombination, evading antibodies; viruses lack this programmed genomic switching.​

• c. Antigenic drift: Influenza viruses accumulate point mutations in hemagglutinin (HA) and neuraminidase (NA), leading to seasonal epidemics by incrementally altering epitopes.​

• d. All of the above: Incorrect, as viruses cannot employ true antigenic switching, which requires complex eukaryotic gene expression sites absent in viral genomes.​

Viral Antigenic Changes

Pathogens alter surface antigens to dodge antibodies and T-cells. Antigenic shift involves segmented RNA viruses reassorting genes in co-infected cells, producing hybrids unrecognizable to immunity. Drift causes minor HA/NA mutations from RNA polymerase errors, enabling yearly flu vaccine updates.​

Distinguishing Switching

Antigenic switching features programmed VSG gene activation in trypanosomes, with ~2500 variants expressed sequentially via telomeric sites. Viruses rely on mutation/reassortment, not this recombination-based toggle.​