Most
viruses (e.g. influenza and many animal viruses) have
viral envelopes covering their protective protein
capsids. The envelopes are typically derived from portions of the host
cell membranes (
phospholipids and proteins), but include some viral
glycoproteins. They may help viruses avoid the
host immune system. Glycoproteins on the surface of the envelope serve to identify and bind to
receptor sites on the host's membrane. The
viral envelope then fuses with the host's membrane, allowing the capsid and viral
genome to enter and infect the host.