Molecular model of the envelope (Env) spike protein from the human immunodeficiency virus (HIV) complexed with antibody fragments (lilac). This protein mediates the entry of the virus into host cells. The receptor domain (red) binds to proteins on the host cell, allowing the fusion domain (yellow) to fuse with the host cell membrane. The protein is covered in sugars (blue), known as a glycan shield, that help it to evade the immune system.