Haemagglutinin is an antigenic glycoprotein which causes erythrocytes to clump together. It is responsible for The influenza virus is known for its ability to mutate. This new strain appears to be a result of reassortment of human influenza and swine influenza viruses, in all four different strains of subtype H1N1. Reassortment is the mixing of genetic material of a species into new combinations in different individuals. This process has been responsible for some of the major genetic shifts experienced in previous pandemics. This novel strain of the H1N1 Influenza A subtype is a result of a mixing of the virus from humans, birds and pigs.