Hemagglutinin is a surface glycoprotein used by several viruses to bind to sialic acid receptors on host cells (including red blood cells, hence the name “hemagglutinin” due to its ability to agglutinate RBCs in vitro), facilitating viral entry.

• Influenza Virus: Has HA for cell attachment; key target for vaccines and subject to antigenic drift/shift.
• Measles Virus: Has H protein (hemagglutinin) for attachment. (MMR vaccine component).
• Mumps Virus: Has HN protein (hemagglutinin-neuraminidase) for attachment and release. (MMR vaccine component).
• Parainfluenza Virus: Has HN protein (hemagglutinin-neuraminidase); common cause of croup.
• Rubella Virus: E1 glycoprotein has hemagglutinating activity, involved in attachment. (MMR vaccine component, relevant for CRS).

Also notable (Bacterial):

• Bordetella pertussis: Produces Filamentous Hemagglutinin (FHA) adhesin, important for whooping cough pathogenesis and vaccine component.