has an inner core consisting of a protein capsid that encloses two molecules of RNA, two molecules of an enzyme called reverse transcriptase, and a few other enzyme molecules. The capsid is enclosed in another layer of viral protein, the matrix. External to this is a viral envelope composed of phospholipids and glycoproteins derived from the host cell. Like other viruses, HIV can be replicated only by a living host cell. It invades helper T (CD4) cells, dendritic cells, and macrophages. HIV adheres to a target cell by means of one of its envelope glycoproteins and “tricks” the target cell into internalizing it by receptor-mediated endocytosis. Within the host cell, reverse transcriptase uses the viral RNA as a template to synthesize DNA—the opposite of the usual process of genetic transcription. Viruses that carry out this RNA → DNA reverse transcription are called retroviruses.28 The new DNA is inserted into the host cell’s DNA, where it may lie dormant for months to years. When activated, however, it induces the host cell to produce new viral RNA, capsid proteins, and matrix proteins. As the new viruses emerge from the host cell (fig. 21.31b), they are coated with bits of the cell’s plasma membrane, forming the new viral envelope. The new viruses then adhere to more host cells and repeat the process.