HIV entry into a host cell. Highly schematic diagram of viral entry. The trimeric gp120 subunit of the viral envelope glycoprotein complex (Env) binds CD4 on the target cell surface, triggering a conformational change that promotes interactions with chemokine receptors and activates the trimeric transmembrane subunit gp41 to mediate membrane fusion, so that the viral contents can enter the cell. Reproduced from DeFranco AL, Locksley RM, Robertson M: Immunity: The Immune Response in Infectious and Inflammatory Disease. London: New Science Press; 2007, with permission of Oxford University Press.