Final answer:
HIV uses the glycoprotein gp120 to attach to the CD4 receptor on T cells, which is a critical step for the virus to infect the cell.
Step-by-step explanation:
The human immunodeficiency virus (HIV) attaches itself to receptor sites by means of the glycoprotein gp120. This glycoprotein binds to the CD4 receptor, a glycoprotein on the surfaces of T cells, which is a crucial step in the infection process. The CD4 molecule is a type of cell adhesion molecule important for immune response and is normally found on the surface of T lymphocytes. Following this initial binding, subsequent steps lead to the fusion of the viral envelope with the cell membrane and entry of the viral contents into the cell.