Hemagglutinin is a type of protein found on the surface of certain viruses, including the influenza virus. It plays a crucial role in the virus's ability to infect host cells by binding to specific sugars on the surface of these cells. This binding allows the virus to enter the cell and replicate. In addition to its role in infection, hemagglutinin is also important for the immune response. The body can produce antibodies against hemagglutinin, which helps to neutralize the virus and prevent illness. This is why hemagglutinin is a key target in the development of vaccines for influenza.