Influenza A and B virus hemagglutinin protein (HA)

Hemagglutinin is the major surface glycoprotein on both influenza A and B viruses, playing a critical dual role in **viral attachment** and **entry into host cells**. Each viral particle displays multiple trimeric HA spikes, each consisting of three identical subunits. HA binds to sialic acid residues on host cell surfaces, facilitating virus entry by **receptor-mediated endocytosis**. Upon exposure to endosomal low pH, HA undergoes a large conformational change, causing fusion between the viral envelope and the host endosomal membrane, a step essential for infection[5][2][6]. HA is synthesized as a precursor (HA0) that is proteolytically cleaved into HA1 and HA2 subunits by host enzymes, creating a fusion-competent molecule[2][5][1]. HA's antigenic properties define influenza subtype and strain, and its head domain is the major target of neutralizing antibodies, making it a primary focus for vaccine design and monoclonal antibody therapy[2][5]. The protein is highly variable (especially in the receptor binding domain), leading to immune escape and frequent need for vaccine updates[1][6][5]. Therapeutic inhibition of HA is pursued with monoclonal antibodies, but current direct-acting antivirals primarily target neuraminidase or polymerase, rather than HA itself; however, HA is a major **therapeutic target for vaccine-induced immunity and passive antibody therapies**[5][2].

Other names

hemagglutinininfluenza hemagglutininflu HAviral hemagglutinin

Inhibition of virus binding to sialic acid receptors (by neutralizing antibodies); Inhibition of membrane fusion activity (by antibodies); Prevention of viral entry into host cells