Questions: Antigenic Variation and Immune Evasion by Pathogens

Question 1

A new influenza strain emerges with a completely novel hemagglutinin subtype that no living human has ever been exposed to, causing a pandemic with very high attack rates. This most likely resulted from:

• A. Accelerated antigenic drift — many point mutations accumulated rapidly in the hemagglutinin gene
• B. Antigenic shift — two different influenza strains co-infected the same cell and exchanged entire genome segments, producing a novel subtype
• C. Molecular mimicry — the virus adopted surface proteins resembling a common human antigen
• D. VSG switching — the virus expressed a new variant surface glycoprotein from its gene library

Answer: (B)

A completely novel hemagglutinin subtype that the entire human population lacks immunity to is the signature of antigenic shift — reassortment of whole genome segments between co-infecting viral strains. Drift (gradual point mutations) generates incremental changes that partially escape existing immunity, causing seasonal epidemics in *partially* immune populations. Shift generates entirely new subtypes that outrun *all* existing immunity, causing pandemics. The 1918, 1957, 1968, and 2009 pandemics all involved reassortment events. VSG switching is a trypanosome mechanism, not influenza.

Question 2

Why does the influenza vaccine require annual reformulation, unlike vaccines for measles or polio that provide lifelong protection?

• A. The influenza vaccine is made of live attenuated virus that degrades over one year
• B. Antibody levels from flu vaccination naturally decline to zero within one year
• C. Antigenic drift continuously accumulates point mutations at antibody-binding sites on hemagglutinin, so last year's strain no longer matches this year's circulating strain
• D. Influenza mutates its entire genome every year through reassortment, making all previous immunity irrelevant

Answer: (C)

Antigenic drift is the mechanism: RNA polymerase lacks proofreading, generating high error rates. Mutations that cluster at antigenic sites — the exposed loops where antibodies contact hemagglutinin — provide selective advantage by slightly altering the epitope shape. Antibodies from last season's infection or vaccine no longer fit precisely enough to neutralize the new variant. This is distinct from shift (whole segment reassortment). Measles and polio viruses are antigenically stable — their surface proteins are constrained by functional requirements — so vaccines remain effective indefinitely.

Question 3

Mutations in influenza hemagglutinin that enable immune escape occur randomly across the entire protein, with equal probability at any amino acid position.

• T. True
• F. False

Answer: False

This is incorrect. Antigenic variation is not uniformly random — mutations cluster at specific *antigenic sites*, the exposed surface regions where antibodies physically contact the protein. Mutations in these regions alter the epitope's shape, preventing antibody recognition. Mutations in structurally buried or functionally constrained regions are generally neutral with respect to immune evasion and may be lethal if they disrupt hemagglutinin's critical function (binding to host cell sialic acid receptors). The non-random clustering of escape mutations at antibody-contact sites is why tracking specific antigenic sites is central to influenza surveillance.

Question 4

Antigenic drift and antigenic shift differ fundamentally in their mechanism: drift involves gradual accumulation of point mutations while shift involves exchange of entire genome segments between co-infecting strains.

• T. True
• F. False

Answer: True

This distinction is critical for epidemiological prediction. Drift produces variants that partially escape existing immunity — enough to cause seasonal epidemics in populations with residual immunity from prior exposure. Shift produces variants with an entirely new antigenic profile — no population has any immunity — which is why shift events are associated with pandemics rather than seasonal outbreaks. Understanding the mechanism explains why public health can manage drift through annual vaccine updates, while shift events require emergency pandemic responses.

Question 5

Explain how trypanosomes can maintain a chronic infection despite the host mounting repeated adaptive immune responses against them.

Model answer: Trypanosomes maintain a library of hundreds of genes encoding variant surface glycoproteins (VSGs) and systematically switch which VSG gene is expressed. When the host mounts an antibody response that clears most of the current parasite population, a small subset that has already switched to a new VSG variant survives and expands. The immune system then generates antibodies against the new variant — but by then, another small subset is already switching again. This keeps the parasite perpetually one step ahead of adaptive immunity, sustaining chronic infection indefinitely. The immune system cannot catch up because the target is continuously moving.

This mechanism is qualitatively different from influenza's antigenic drift. Drift is population-level evolution over time. VSG switching is individual-level gene expression switching within a single infection. The trypanosome does not wait for mutations — it has pre-programmed genetic diversity ready to deploy. This explains why despite decades of research, there is still no vaccine against African sleeping sickness: any vaccine targeting one VSG variant would be outflanked by the parasite's gene library.