sentences:  Gp120 is only exposed when in close proximity to a host cell and the space between the viral and host cell membranes is small enough to sterically hinder the binding of antibodies.The glycoprotein gp41 is non-covalently bound to gp120, and provides the second step by which HIV enters the cell.It is originally buried within the viral envelope, but when gp120 binds to a CD4 receptor, gp120 changes its conformation causing gp41 to become exposed, where it can assist in fusion with the host cell.