The cells of the immune system are the targets of HIV. Dendritic cells are the first cells to be infected, which is critical as dendritic cells induce the primary immune response. These cells present HIV antigens to CD4 and CD8 cells, primarily the CD4 subset of T-lymphocytes, leading to activation and an increase in HIV production. The CD4 receptor serves as a receptor for the virus. Macrophages are also infected by HIV and HIV may enter the brain via circulating T-cells and macrophages.
The joining of HIV and the host cell involves a spike on the HIV envelope and a CD4 molecule. 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.