what are the three major antigenic types of hemagglutinins that have been recognized in human influenza viruses?
<answer> H1, H2 and H3 are the three major antigenic types of hemagglutinins that have been recognized in human influenza viruses. <context> Indeed, hemagglutinin is the major virulence (disease-causing) factor of the influenza virus. There are three distinct haemagglutinins important in human infections that are encoded by genes in the virus. These are designated as H1, H2, and H3. Animal influenza viruses contain nine additional types of hemagglutinin. Neuraminidase is the common name for acetyl-neuraminyl hydrolase.