Influenza virus attachment – Identify the glycoprotein: In influenza (an enveloped orthomyxovirus), which surface protein primarily mediates attachment to host cell receptors?

Introduction:
Successful viral infection begins with attachment to host cells. For influenza viruses, two major glycoproteins decorate the envelope: hemagglutinin (HA) and neuraminidase (NA). Distinguishing the primary roles of HA and NA is essential in understanding pathogenesis, antiviral targets, and vaccine design.

Given Data / Assumptions:

• Influenza is an enveloped RNA virus with HA and NA spikes on its surface.
• Attachment requires recognition of sialic acid on host cell glycoproteins/glycolipids.
• The question asks which protein is responsible for attachment, not release.

Concept / Approach:
HA binds to terminal sialic acid residues (e.g., alpha-2,3 or alpha-2,6 linkages), initiating entry by receptor-mediated endocytosis. NA acts later to cleave sialic acid, facilitating virion release and preventing self-aggregation. Therefore, the correct attachment mediator is HA.

Step-by-Step Solution:
Step 1: Identify influenza surface proteins: HA and NA.Step 2: Match function: HA → receptor binding; NA → sialidase activity aiding virion egress.Step 3: Recognize that fimbriae/flagellae are bacterial structures, not viral.Step 4: Conclude hemagglutinin is the attachment protein.

Verification / Alternative check:
Vaccine strain naming (e.g., H1N1, H3N2) emphasizes HA subtype for antigenicity and receptor binding properties, underscoring HA’s role in attachment.

Why Other Options Are Wrong:

• Fimbriae/Flagellae: present in bacteria, not in viruses.
• Neuraminidase: cleaves sialic acid to aid release and spread; not the primary attachment factor.
• M2 ion channel: involved in uncoating and pH regulation inside endosomes.

Common Pitfalls:
Confusing functions of HA and NA; assuming any surface protein must be for attachment.

Final Answer:
Hemagglutinin.