The basic structure of HIV is similar to other viruses (Figure 1). HIV has a core of genetic material surrounded by a protective sheath, called a cap Sid. The genetic material in the core is RNA, ribonucleic acid, which contains the information that the virus needs to reproduce and perform other functions. You can think of RNA as the set of rules the virus follows in order to live.

In HIV, viral RNA has a protein called "reverse transcriptase" that is crucial for viral replication inside of T-cells. The function of reverse transcriptase, which means "writing backwards," will be explained later when we explain how HIV infects the T-cell.

HIV, like all other viruses, has proteins that are particular to itself. These proteins are called antigens. Antigens have diverse functions in viral replication. In the case of HIV, a combination of two antigens, the gp120 and the gp41, allow the virus to hook onto T-cells and infect them. These antigens are located on the surface of the virus. (Another HIV antigen is the p24, an antigen of the core of the virus that is measured to estimate the amount of active free-floating virus in the blood of HIV positive people).