The joining of the HIV and the host cell involves a spike on the HIV envelope and a CD4 molecule on the T-lymphocyte, macrophage, or brain cell.
The molecule on the HIV spike is called glycoprotein 120 or gp120. The "120" refers to the molecular weight of the glycoprotein.
While the CD4 site is important in viral binding, there is evidence that there are other molecules called co-receptors also involved.
These molecules are embedded in the membranes of T-lymphocytes, macrophages, and brain cells. In the T-lymphocyte the abbreviated name of the protein molecule is CXCR4.