Questions: Viral Attachment Proteins and Receptor Binding

Question 1

A novel virus is isolated and found to infect only cells lining the upper respiratory tract, even though the virus circulates briefly through the bloodstream. What best explains this tissue-specific pattern of infection?

• A. The virus is destroyed by immune cells before it can reach other organs
• B. The virus's attachment proteins specifically bind a receptor expressed primarily on respiratory epithelial cells, restricting which cell types can be infected regardless of viral circulation
• C. Respiratory cells divide more rapidly, giving the virus more opportunities to infect them
• D. The virus is adapted to the lower temperature of the respiratory tract and cannot replicate elsewhere

Answer: (B)

Tissue tropism — which cell types a virus can infect — is primarily determined by the molecular specificity of the viral attachment protein for its receptor. If the receptor is concentrated on respiratory epithelial cells (as ACE2 is in SARS-CoV-2 infection), the virus can only initiate infection where that receptor is present, even if the virus transiently reaches other tissues. Temperature adaptation and immune clearance can also affect tropism, but receptor specificity is the primary molecular determinant.

Question 2

Why do mutations in viral attachment proteins sometimes allow a virus to escape existing immunity while still remaining infectious?

• A. The mutations inactivate the attachment protein, so the immune system no longer recognizes it as foreign
• B. Mutations shift viral replication to intracellular organelles where antibodies cannot reach
• C. Specific mutations in the attachment protein can alter its surface shape enough to prevent antibody binding while preserving the core receptor-binding interaction
• D. Mutations increase viral replication speed, allowing the virus to outpace immune responses

Answer: (C)

Neutralizing antibodies bind to specific epitopes (surface regions) on the attachment protein and physically block receptor interaction. If a mutation changes the shape of those antibody-binding epitopes, existing antibodies can no longer lock onto the protein — immune escape. But the receptor-binding site is under opposite pressure: mutations that prevent receptor binding eliminate viral infectivity. The attachment protein must thread this needle — accumulating mutations at antibody-binding sites while preserving the receptor-binding domain. This is the molecular mechanism of antigenic drift and the reason new vaccines are periodically needed.

Question 3

Neutralizing antibodies protect against viral infection by entering infected cells and degrading viral genetic material before it can replicate.

• T. True
• F. False

Answer: False

Neutralizing antibodies work extracellularly, not intracellularly. They bind to viral attachment proteins (like spike protein or hemagglutinin) on the virus surface and physically block the attachment protein from interacting with the host cell receptor — preventing viral entry in the first place. This is why they are called 'neutralizing': they neutralize the virus's ability to attach and enter before infection is established. Antibodies cannot penetrate into infected cells to destroy viral RNA (that is the role of cytotoxic T cells and intracellular innate immune mechanisms).

Question 4

The tissue tropism of HIV for helper T cells is determined by the specific binding of HIV's gp120 attachment protein to the CD4 receptor expressed on those cells.

• T. True
• F. False

Answer: True

This is the direct mechanistic explanation for why HIV specifically depletes helper T cells rather than, for example, infecting liver cells or neurons. gp120 binds CD4 (and a co-receptor, CCR5 or CXCR4) with high specificity. Cell types that lack CD4 cannot be infected by HIV regardless of viral exposure. The immunological devastation of AIDS follows directly from this receptor specificity: helper T cells are the coordination hub of the adaptive immune system, and their depletion progressively collapses immune function.

Question 5

Why do influenza vaccines need to be reformulated and administered annually, in terms of the biology of viral attachment proteins?

Model answer: Influenza's hemagglutinin (HA) is both the attachment protein (binding sialic acid receptors on respiratory epithelial cells) and the primary target of protective neutralizing antibodies. HA undergoes continuous antigenic drift — point mutations in its surface-exposed regions that gradually alter the epitopes recognized by existing antibodies. Antibodies generated by vaccination or prior infection are shaped to the old HA surface; mutated HA may fit host receptors equally well (maintaining infectivity) while no longer matching the antibody binding sites (enabling immune escape). Because HA is simultaneously under evolutionary pressure to maintain receptor binding AND to escape immune surveillance, the dominant circulating strains change year to year. New vaccines must match the HA surface of the predicted circulating strain to generate antibodies that can block attachment to host receptors.

This question requires connecting three concepts: (1) HA's dual role as attachment protein and antibody target, (2) the evolutionary pressure on attachment proteins to escape immunity while maintaining receptor binding, and (3) the practical consequence for vaccine design. The key insight is that the attachment protein's surface exposure — which makes it an excellent vaccine target — is also what makes it the evolutionary bull's-eye for immune escape mutations.