While more research is needed to confirm the sequence, it is believed that the gp120 molecule on HIV unites, via the V3 loop, with the CD4 molecule on the T-lymphocyte, macrophage, or brain cell. The gp120 molecule then changes shape, drops down, and binds with the co-receptor. Thus, HIV binds twice to the T-lymphocyte or macrophage - once at the CD4 molecule and once at the co-receptor site.
