Hemagglutinin

Hemagglutinin is an influenza virus surface glycoprotein that binds sialic acid on host cells and mediates viral membrane fusion after uptake.

Sialic acid: A family of acidic sugars commonly found at the ends of cell-surface glycans. Hemagglutinin binds sialic acid-bearing glycans to attach influenza virions to cells.
Influenza A virus: A segmented, negative-sense RNA virus that infects humans and many animal species. Hemagglutinin is one of its two principal surface glycoproteins.
Influenza vaccine: A vaccine designed to induce immunity against influenza viruses. Many influenza vaccines present hemagglutinin to elicit neutralizing antibodies.
Neuraminidase: An influenza surface enzyme that cleaves sialic acid from host glycans and newly formed virions. Unlike hemagglutinin, neuraminidase promotes release of progeny virions from infected cells.
Receptor-mediated endocytosis: A cellular uptake process in which surface receptors help bring bound material into the cell. After hemagglutinin attaches, influenza commonly enters cells through this uptake pathway.
Viral envelope: A lipid membrane surrounding some viruses, derived from a host-cell membrane during budding. Hemagglutinin is embedded in this membrane and projects from the virion surface.
Hemagglutination inhibition assay: A serological test that measures antibodies able to block virus-mediated red-cell agglutination. It estimates antibody responses that interfere with hemagglutinin binding.
Influenza polymerase: The viral enzyme complex that transcribes and replicates influenza RNA. Hemagglutinin mediates entry at the cell surface, whereas polymerase operates on viral genomes inside cells.
Endosome: A membrane-bound compartment that sorts material internalized by cells. Its acidic interior triggers hemagglutinin to change shape and drive membrane fusion.
Glycoprotein: A protein with covalently attached carbohydrate chains. Hemagglutinin's glycans can affect its stability, shielding, and antigenic properties.

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