Ribbon representation of the hemagglutinin HA0 trimer from the 1918 influenza virus. Each monomer (colored differently) possess two important sites: 1) the 'Receptor binding site' for virus attachment to the host lung epithelial cells via sialic acid containing host cell receptors. 2) the 'Cleavage site' where for full infectivity, the single chain (HA0) is cut into two chains (HA1 & HA2). At the N-terminal end of the HA2 chain is the fusion peptide which is critical for subsequent membrane fusion events that lead to infection.