Hemagglutinin is a 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. Hemagglutinin binds to sialic acid residues on the surface of cells, facilitating the entry of the virus into the cell. This protein is also important for the immune response, as it can trigger the production of antibodies. Vaccines for influenza often target hemagglutinin to help the body recognize and fight the virus. Understanding hemagglutinin is essential for developing effective treatments and vaccines against viral infections.