Influenza A virus hemagglutinin antigen (H1N1 subtype) (HA (H1N1) or H1 hemagglutinin)

**Influenza A virus hemagglutinin antigen (H1N1 subtype)** is the principal surface glycoprotein of the H1N1 influenza A virus, functioning as both the receptor-binding protein and fusion protein[4][5]. HA (H1N1) is a homotrimeric glycoprotein composed of HA1 (globular head, receptor binding) and HA2 (stem, fusion) subunits, generated by post-translational proteolytic cleavage of a precursor (HA0)[1][6][7]. The HA1 subunit is responsible for attachment to sialic acid receptors on host respiratory epithelial cells, while the HA2 subunit mediates fusion of the viral envelope with the endosomal membrane after endocytosis[1][2][4][5]. Hemagglutinin is highly immunogenic and defines the viral subtype (e.g., H1 in H1N1), being the main target of neutralizing antibodies, vaccines, and serological diagnostics[2][3][5]. HA undergoes frequent mutation (antigenic drift) and less frequent reassortment (antigenic shift), driving influenza evolution, immune escape, and pandemic risk[3][5][9]. Annual influenza vaccines contain updated HA components to match circulating strains, and experimental universal vaccines focus on conserved HA regions[5][7]. Broadly neutralizing antibodies targeting conserved HA epitopes are under investigation as therapeutics and as models for next-generation vaccines[5][7]. The term "hemagglutinin" derives from the protein's ability to cause agglutination of red blood cells, which underlies traditional laboratory assays[4][5]. Note: While antivirals such as oseltamivir and zanamivir target neuraminidase, not hemagglutinin, HA is the central antigen in influenza vaccine development and the main focus for monoclonal antibody therapies.

Other names

Influenza A hemagglutinin (H1 subtype)H1 hemagglutininHA (H1N1)Influenza A H1 hemagglutinin antigen

Vaccines induce neutralizing antibodies that block receptor binding or membrane fusion[2][5][7]; Monoclonal antibodies bind HA head or stem, blocking viral entry[5][7]; Inhibition of HA-mediated attachment to sialic acid receptors; Inhibition of conformational changes required for membrane fusion