Hemagglutinin (HA) is a class I viral fusion protein from Influenza virus. It is a major glycoprotein, comprising over 80% of the envelope proteins present in the virus particle. HA binds to sialic acid-containing receptors on the cell surface, bringing about the attachment of the virus particle to the cell, and is responsible for penetration of the virus into the cell cytoplasm by mediating the fusion of the membrane of the endocytosed virus particle with the endosomal membrane. The extent of infection into host organism is determined by HA. In natural infection, inactive HA is matured into HA1 and HA2 outside the cell by one or more trypsin-like, arginine-specific endoproteases secreted by the bronchial epithelial cells. The HA protein is a homotrimer of disulfide-linked HA1-HA2. It also plays a major role in the determination of host range restriction and virulence. Genetic variation of hemagglutinin and/or neuraminidase genes results in the emergence of new influenza strains.