HIV has a core of genetic material surrounded by a protective sheath, called a capsid. 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. 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. Let's look at the structure of the T-cell. T-cells are the main target of HIV in the blood, and act as the host that the virus needs to replicate. The cell's DNA has all the information that the cell needs to function.