CD4 and infection: Infection with HIV, a lentivirus [4], is possible due to interaction of the envelope (Env) glycoproteins present on the surface of the virus (i.e., gp120 and gp41) with, respectively, the receptor CD4 and the chemokine co-receptors (CXCR4 and CCR5) located on the primary target cells (activated CD4+ T cells).